This is just a little idea; I've often speculated about the inner workings of the Administratum, the incomprehensively massive and truly byzantine organization responsible for the administration of the majority of the Imperium's assets, as the name implies. However, seeing as 40k is a wargame, and the Administratum is as much about correct filing of tax reports (78b in triplicates, yellow through blue and green to the clerk on Labyrinthine IV) and demography as it is about troop transport allocation, it's not an organization that receives a lot of attention despite its relatively huge importance in the grand scheme of things, understandable though it may be.
Anywho, I thought people who were interested could come together and - for fun - outline various organs, agencies and departments of the Administratum, and a bit about what they do. This is of course going to be fan material (although feel free to bring up official sources as well), and as such, we are going beyond the canon, and may well disagree on a number of things. That's fine.
Here's my own little attempt;
Departmento Internum:
- Internal functions department. Jealously keeps its own records of the assets and procedures of various other departments, and has a right to request samples or specific documents for control. Heavily associated with (and occasionally subservient to) the Estate Imperium (the Imperium's main record-keeping division.) Its head is the Comptroller-Internal.
- Notable subdivisions:
--- The Officio Investigatorum: The investigating branch of the Internum. Its members possess limited prosecutoral powers within the Administratum, but does not concern itself with the wider Imperial society, nor the canons of Imperial Law, nor does it possess the judicial powers of the Adeptus Arbites. Admittedly it's authority over certain divisions, most notably the Departmento Munitorum, is highly limited. Its main purpose is to investigate already-suspicious activities and to uncover procedural misconduct and breach of clearances or other established practices. It also serves, in some limited capacity, to prevent Ruinous influences, so as to prevent the oft-destructive attention of the Inquisition. In cases where there are clear breaches of Imperial Law, (such as when plans of secession are uncovered) or xenos or daemonic taint are a fact, the Investigatorum relinquishes all authority to the relevant bodies and instead enters damage-assesment mode, where it attempts to limit losses and account for what is irrevocably lost.
--- The Officio Vestiario: The Imperial Wardrobe. In ancient times responsible for directing the designs of imperial uniforms in general, its current purpose is to maintain and revise the rules of the robes and honors of the various Administratum departments, as well as specific items related to the various local entities, often mired in millennia of confusing, sometimes contradictory, yet highly sensitive and specific traditions. One of the smaller subdivisions, but its ancient prestige is hard-dying.
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Comment if you will, or make up one of your own. It'd be fun to see if we could make something coherent out of this, so it'd be great if we all try to envision how the various departments (and even extra-departmental) organizations work with regards to each other and the greater imperium. Taking inspiration from real-life administrations might be a good idea.