The number of BMUs (building maintenance units) is the total number of operable mechanical devices installed on a building which move systematically over the facade or other surfaces (up and down and/or side to side) while carrying either human window washers or mechanical robots to clean, repair, or otherwise maintain the facade, windows, large-span ceilings, and/or other building surfaces. These units are usually stored on or near the roof, and suspended over the edge of the roof when used.

The count concerns installed, operable maintenance units. A unit can carry workers or robotic cleaning equipment; it need not be used only for window washing.

Maintenance of other facade surfaces and large-span ceilings is also included. Roof storage and suspension over a roof edge are usual arrangements rather than requirements for every unit.