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Unlike the modern 40k forge worlds, which are little more than factory planets living within imperial sectors and sometimes claimining a mining moon or industrial world here or there, the Mechanicum of the Crusade was a true real empire unto itself. It included its own political, industrial, and religious institutions all built around a feudal structure in much the same way the Imperium was, with Segmentum, Sector, Subsector, System, Planetary, and Hive governance. It was more self contained, not having to import food or manpower from the Imperium but self reliant.
You can see the mimicry in Imperial Command structure too:
Archmagos Intendants = Planetary Governors, who are subservient to Fabricator Generals from powerful forge worlds acting as their own segmentum command
The Basilikon Astra = Imperial Navy
The Prefecture Magisterium = The Ecclesiarchy/Inquisition
and so on
The picture explains the structure more precisely
