Almost all rotating equipment has peripheral projections that define the Operating Envelope diameter.
When a rotating machine operates, components such as tool holders, tool bit feed mechanisms, etc. may stick out past the principal diameter of the frame. When a machine is at rest, the parts that stick out are not so apparent. Care must be taken to ensure that there is enough clearance between the operating envelope and obstructions such as other pipes, bulkheads, walls, machinery, ropes and cables.
Needless to say that if an operator was positioned inside the clearance of a machine at rest and rotation started, it could be extremely hazardous when the rotating elements approach and pass by tight clearance spaces. Always check and make sure the Operating Envelope is safe and clear.