An O/M manual
from a single vendor
for a specific piece of equipment, detailing the construction and materials used in its construction
. Basically, a HOWTO Build _____
manual. Maintenance personnel most often use these when maintaining or repairing a certain piece of equipment. There are also O/M manuals for things like paint, floor tiles, desks, ceiling tiles, etc. These types are rarely referenced, and if they are, it's to get the specific color of an item for replacement. They are also used rarely to find historical details (i.e. seeing what kind of equipment they used 100 years ago, and where it was).
O/M manuals also include wiring diagrams for M/E equipment and other pertinent information to expedite the repair of crucial equipment such as furnaces and heating plants up here in Minnesota.
A particularly useless variety is the structural steel shop drawing. These are utilized in the construction of a building by the company manufacturing structural steel (!) for a building. Often these are girders and framing, like the joists in your ceiling and the studs in your walls, only made of steel and really big instead of wood 2x4's. These are useless because once a large steel girder is cut to the precise length and assembled, it is encased by cement. This cement makes it a real bitch to save old materials when moving a wall or expanding the building.