The Organisation of the Imperial Forces
There are a number of sources regarding the military structure of the Imperium and no doubt many have there own opinion as to what was what at the time.
Like the suggested organisation of the Legio Astartes this thread offers suggestions for the organisation of the Imperial Army based on what information has been presented using the Codex Imperial Guard as an army list.
The Expeditions
The great crusading armies of the Legions are organised into large fleets of vessels called Expeditions.
Imperial forces within those fleets under the direct command of the Astartes Legions, presenting each expeditionary commander whether he's an Astarte Captain or a mighty Primarch with a vast army at his disposal.
The Imperial Army
Imperial armies are often little more than loosely organised large groups of soldiers. Each regiment is assigned a warship which often holds around a thousand Imperial Soldiers and their equipment from the humble las-gun to the mighty tanks of the army.
Where the Imperial Commander in charge of a Regiment answers directly to the Astartes the ship that transports his men comes directly under his command.
Designations
The Imperial army uses two prefixes to the army name to firstly show which expedition they belong to and secondly show their regiment number. This is then followed by the Regiment's name.
So for example, the 32nd/102nd Herek's Hellcats would be the 102nd regiment assigned to the 32nd Expedition.
Expeditions can vary in size from a single company of Astartes supported by a couple of Imperial Army Regiments to an entire legions supported by a hundred Imperial Guard Regiments, Titans and other military orders like the Sisters of Silence.
Using the Codex Imperial Guard
Players can use the Imperial Guard codex to represent their own Imperial Regiments with what ever Doctrines they see fit to best represent the Regiment.