This gives the bay, or chassis sub-unit, location of the physical port where this MAC address entry was learned, part of the bay/shelf/slot/port hierarchy used to physically locate ports on larger chassis systems. An admin doing a physical hands-on investigation of a suspicious device would use this field, together with the shelf, slot, and port ID fields, to walk directly to the exact patch panel or line card location rather than guessing from a bare port number. A value of 2 would mean the port is physically located in bay 2 of the chassis.