From Halopedia, the Halo wiki
The Assembly Forges refer to the various central manufacturing facilities of the Covenant, individually referred to as an assembly forge. These facilities produced items piece-by-piece using clouds of programmable nanomachines. These nanomachines were fed instructions via a design pattern, maintained by artisan-armorers. An imperfect replication of the assembler vats once used by the Forerunners, assembly forges are susceptible to mutations in design patterns, unaccounted variance in feedstock material, and transcription errors. The mutation of a design pattern often required the resetting of the forge that had attempted to produce the design.
The Assembly Forges produced numerous matériel—ranging from weapons, vehicles, shuttles, and starships—for the Covenant war machine. Due to advanced manufacturing methods applied by fabrication plants like Assembly Forges, the Covenant deliberately avoided rigid standardization and mass production of their starships. While considered a separate entity from the Assembly Forges, the Ministry of Preparation's Sacred Promissory operated assembly forges.
After the fall of High Charity during the Great Schism in November 2552 and the loss of the Primary Assembly Forges, the Swords of Sanghelios and successor factions of the Covenant were forced to turn to independent forges and armories for equipment, notably Lodam Armory.
There were three known types of assembly forge facilities, graded based on their size:
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