system
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human
[Warhammer 40k] So what does an STC look like?

Is it some giant factory, or something like a construction guide?
gpt
No one knows. We know it's a combined assembler system and database, but the exact form they took is unknown. There have been some discoveries which indicate that they didn't have a set form, but were really a collection of various automated manufacturing systems for different purposes (IE a Rhino Assembler was different than a Men of Iron assembler).

There is some speculation that the Ark Mechanicus ships are the 'Base' form of the STC system. Massive ship-sized factories.