Anyway, the regiments of the Astra Militarum are officially known as the Militarum Regiment. This refers not to a single "regiment" but pretty much all the regiments that come from a single world (or system or sector or even voidborn, depending on how the "Militarum Regiment" is organized or how they recruit).
Each Militarum Regiment comprises multiple regiments, all of whom come from the same planet. For example, all Guardsmen from Cadia as belonging to the Cadian Regiment, or all Guardsmen from Ryuusei as belonging to the Ryuusei Regiment. On their home worlds, the forces who serve to defend the planet may be split into battalions, divisions, cohorts, militia groups, geno-corps and other formations, but in the Imperial Guard, they are all simply referred to as different types of regiments.
Each Astra Militarum Regiment typically follows a core composition - for example, infantry regiments will obviously be mostly infantry with very little artillery, whereas tank regiments won't have much infantry (apparently, in the 41st Millennium, armor infantry - otherwise known as mechanized infantry, have their own classification instead of being under armor regiments). This was laid down by the then Lord Commander Militant of the Imperium, in the aftermath of the Horus Heresy to prevent any one commander from having access to diverse assets to wage war against the Emperor and the Imperium like what happened during the 31st Millennium.
Typically, this means that when waging war, several different regiments have to work together. Unlike the Space Marine Chapters, which are self-contained forces capable of executing a campaign by themselves without necessarily requiring support, the Imperial Guard must muster diverse assets from various regiments, combining infantry, tanks and artillery from ideally different worlds to prevent any sort of politically grouping. And once that campaign is over, the different regiments are broken up and shipped to different warzones, so as to once again prevent any long-lasting alliances or ties of loyalty from forming between these regiments. So if one regiment turns traitor, the damage will be minimized, and the resources the turncoat commander has will be very limited.
The Gaunt's Ghosts novel series demonstrate the deployment of multiple regiments for a campaign. Dan Abnett's Necropolis, for example, sees the combination of the recon-focused Tanith First and Only light infantry regiment, the aristorcatic and heavy infantry of the 50th Royal Volpone and the tenacious light infantry of the Roane Deepers and the tank regiment of the 1st Narmenian Armored defend Vervunhive on Verghast from the traitorous Zoicans, waves of millions of infantry supported by artillery.
In the subsequent campaign, the Tanith is then deployed alongside the Eighth Pardus Armored to journey to the Shrine of Saint Sabbat on Hagia to evacuate the remains of the Saint, and then they worked together to defend the Shrine from the Infardi. In both cases, infantry and tank regiments cooperate for the campaign before being shipped off to different warzones, and this is typical of most Imperial Guard deployments.
Regiments are typically raised with a strength of several thousand soldiers, with Valhallan infantry regiments numbering over 120,000 men, but there are many differences. The Tanith, for example, only number between four and five thousand men. In contrast, a Vostroyan armored regiment might only have less than 1,500 tank crewmen. Regiments of super-heavy tanks, such as Baneblades and Shadowswords, rarely consist of more than a dozen of these armored behemoths. For example, the 7th Paragonian Super-heavy Tank Company consists of four super-heavy tanks, 40 crewmen, and their associated staff and support personnel that add up to about 160 more men, which also includes Atlas recovery tanks. Given that a tank regiment is normally composed of 3 companies, that means 12 super-heavy tanks for a single Paragonian Super-heavy Tank Regiment. Sounds about right. The point, though, is that though grossly oversimplifying, the Departmento Munitorum classifies regiments based on an equivalent fighting strength or combat effectiveness, though how they measure that is a bit ambiguous. Theoretically, this means 12 super-heavy tanks are equivalent to 120,000 infantry.
Imperial Guard regiments are divided into several companies according to the Tactica Imperium. Depending on the type and size, a regiment may consist of between three and twenty. The Tanith First and Only, for example, has A Company all the way to S, I think? Someone correct me if I'm wrong. Most infantry companies are organized into platoons, typically between three and six. Platoons are usually comprised of a platoon command squad and several ten-men infantry squads, or perhaps twenty-men infantry squads. These are then supported by heavy weapons platoons, perhaps Sentinels and other specialist units.
Non-infantry assets, such as battle tanks, artillery and abhuman squads, may be attached to an infantry company for a battle or the duration of an entire campaign. These aren't permanent additions and are attached as necessary by regimental commanders. Sometimes, armored and artillery regiments are broken down into companies to be seconded to infantry forces, granting heavy firepower to platoons whilst providing meatshields...ahem, troop support for the vehicles.
When an army is assembled, the Departmento Munitorum draws regiments from many different planets, resulting in a conglomeration of uniforms and combat skills instead of a single homogenous force. Again, Dan Abnett's Gaunt's Ghost series often showcase the diverse regiments from multiple worlds working together for single or extended campaigns.
Often, casualties will reduce the overall strength of fighting forces, and while the Departmento Munitorum and the logistics staff of the homeworld will often send new recruits and reinforcements to the depleted regiment, often many of these regiments are fighting so far away from home and have no way of returning or having the fresh recruits reach them, say, in a hotly contested warzone that's being blockaded by the enemy armada. In such cases, the Departmento Munitorum will merge half-strength regiments. For example, the Tanith are often amalgamated with other regiments, such as recruits from Verghast or merged with the Belladon regiment whenever they become understrength. Of course, the Departmento Munitorum will, in such situations, do their best to combine regiments from the same world, such as the 597th Valhallan Ice Warriors, which was formed from combining the 301st Valhallan - all-male specialized planetary assault shock troops - and the 296th rear echelon HQ security and all-female units.
Separate to the regiments from diverse worlds, the Schola Progenium is a more uniform (ha ha, uniform, get it?) program that nurtures orphans of Imperial officials all over the galaxy and train them into becoming Commissars of the Officio Prefectus, elite Tempestus Scion companies of the Militarum Tempestus - or storm-troopers in low Gothic - and the Adepta Sororitas for most of the girls.
As can be seen, a typical infantry company is usually led by a company commander and command squad, and consists of five platoons. Each platoon will have its own platoon commander and command squad, and consists of five infantry squads, though the numbers will inevitably vary according to regiment. They will also have their own transports, though usually limited unless it's a mechanized infantry regiment like the Armageddon Steel Legion, where every single squad will have their own dedicated transport.
Additionally, each platoon may include up to three special weapon squads, five heavy weapon squads and one Conscripts squad, though Conscripts appear to have been fazed out by the 42nd Millennium.
In contrast, a standard Armored Company (and there are a lot of tank companies that don't follow the standard) is composed of a Tank Commander and three or more squadrons of Leman Russ tanks. Again, depending on the world, they might have access to different type of tanks, such as Malcador tanks, Valdor Tank Destroyers, Macharius, Carnodon tanks, and even super-heavy tanks such as Baneblades, Shadowswords and Stormblades.
Other attaches include the preachers of the Adeptus Ministorum, Mechanicus covenants of Tech-priest Enginseers that tend to the tanks and vehicles, abhuman squads from the Militarum Auxilla such as Ogryns and Ratlings, and the Scholastica Psykana's battle psykers, who are the equivalent of a Space Marine's Librarius.
For example, in this infantry regiment, you have a colonel in overall command, and his command squad and presumably support staff forming the command company. Under them, you have 3 infantry companies. The 1st company would have its own company command squad and company commander, typically a captain. Under him would be three platoons, each platoon led by a Lieutenant and his command squad, and consisting of at least three 10-men infantry squads. The 1st platoon would be the veterans, the 2nd platoon also frontline fighters, while the 3rd platoon seems to be tactical reserves and fire support for the other two. The regiment is supplemented by 18 Commissars under a single senior Commissar from the Officio Prefectus, 3 Ogryn squads from the Militarum Auxillia, 83 Preachers and 101 menials hailing from the Adeptus Ministorum and 9 psykers from the Scholastic Psykana.
An armored company is different - the command company is a colonel in a Leman Russ tank (or Rogal Dorn), though he can choose to ride a command Chimera instead or even a Baneblade or super-heavy tank. Under him are three tank companies, each consisting of 10 tanks - a single tank commander leading three squadrons of three Leman Russ tanks. Or Rogal Dorn tanks, given we have that now, or whatever poor tanks that have been banished to Legends after the advent of the 10th Edition Astra Militarum codex and the utter gutting of Imperial Armor units. Supplementing them is a single Baneblade, a "Heavy" company. The armored regiment is supported by a recon squadron of 3 Sentinels and an anti-aircraft squadron of 3 Hydras, assigned to them by overall command.
Terms and naming conventions vary from regiment to regiment, so while the regimental commander is normally a colonel, the military culture from which they're drawn from might use an entirely different term, such as Knight Commander, Chief Hetman, Taisa, etc. Generals could be known as Lord Militants or Shoguns. But despite the varied naming conventions, the ranks are roughly equivalent throughout the regiments.
Often, regiments are combined into combined arms forces where infantry are supported by armor and artillery, but there will be occasions where the mustered forces are ogranized into highly specialized armies designed to meet very specific tactical challenges. For example, a battle group might consist of mechanized infantry and artillery for lightning-fast armored breakthroughs, or an entire Sentinel company assisting light infantry in jungle warfare, or an entirely armored battle group drawing multiple tank and artillery squadrons. Like in the Battle for Tallarn during the Heresy. Or Super-heavy tank squadrons with tank regiment support against Titans.
If necessary, several battle groups will be combined into a huge, operational-level force known as an army group, led by high-ranking commanders such as generals and high marshals. An example of this would be the Sabbat Worlds Crusade in Dan Abnett's Gaunt's Ghosts series, Lord Solar Macharius's conquest of a thousand worlds in 7 years, or even the Indomitus Crusade fleets in current Warhammer 40,000 lore. Millions, even billions of men move at the Warmaster's command, conquering entire sectors.
On the surface, there isn't much to separate the organization of the Astra Militrarum from the Excertus Imperialis, aside from the complete separation of naval assets from Guard forces. Perhaps the Imperial Guard rarely conduct boarding actions and void warfare, but just like the Imperial Army of old, they are divided into specialized regiments (or cohorts) such as recon, infantry, siege or armored. They are commanded by a lord marshal or lord general when mustered in massive battle groups for a campaign, and fight alongside numerous other regiments from disparate worlds. Like Cadians fighting alongside Valhallans and Mordians or the Tanith cooperating with the Vitrian Dragoons and Narmenian Armored in a campaign, the Saturyne Rams and Jovian Grenadiers would battle side by side with Arkadian Janissaries and Lucifer Blacks during the Heresy or Great Crusade.
Maybe another difference would be that the disparate cohorts of an expeditionary fleet will fight together for many years, this becoming a much more unified force than would be the case for the Imperial Guard, whose battle group will be broken up to their individual regiments, each shipped off to a different warzone to prevent the same scenario. Owing to the nature of expeditionary fleets sent to far flung, remote corners of the Galaxy without supply lines or any hopes of reinforcement or reassignment, these cohorts are far more united and loyal to their overall commander than modern regiments, making it difficult for them to consolidate power and establish their own personal armies, to prevent a rebellion on the scale of the Heresy. Furthermore, the distinct separation of the Armada Imperialis from Excertus Imperialis into the Aeronautica Imperialis and Astra Militrarum will deny ambitious generals the resources they need for a coup. Theoretically, anyway, but there are lord commanders and planetary governors who manage to persuade both Guard and Navy forces over to their cause throughout the 10 millennia. Even so, it is incredibly difficult for them to do so within the current structure, and he military assets they have access to are often severely limited.
Also, unlike the Legiones Auxilia of old, modern Imperial Guard regiments do not owe fealty to any Space Marine Chapter, with a few exceptions, such as the Ultramar Auxilia or standing serf armies, or even the Sabbatine forces for the White Consuls. This is to prevent any Astartes Chapter from growing too powerful and becoming a legion again. When the Astral Claws attempted to build their own auxiliary army, they were branded as traitors and ended up leading to a civil war in the Badab Sector.
