OSHA defines a confined space as any space you can bodily enter and work in but has a restricted means of access/egress and isn't designed for continuous occupancy. This definition isn't precise, but OSHA gives examples (e.g., tanks, bins, and vaults) and assumes employers will "fill in the blanks."

OSHA requires employers to identify its confined spaces (and if in doubt, do not leave it out) and apply confined space permitting to it. But as the definition is itself a bit slippery, so can be compliance.

For example, a manager might conclude that, because employees frequently enter a specific pit and it lacks a roof, it's not a confined space. But what happens if someone in another part of the building spills a heavier than air gas (e.g., carbon dioxide) that flows into the pit? The fact that someone failed to properly identify a confined space doesn't mean it isn't one. Assess every space for its potential hazards.