This OID reports the PCI bus number that, together with the device index, uniquely identifies this PCI slot, where values greater than 255 indicate a dummy bus number assigned to an empty slot. An admin auditing PCI slot occupancy can use a value above 255 as a quick signal that a given slot is empty, and on systems with multiple PCI segments the segment number is encoded into bits 8 through 23 of this value. For example, on a system without multi-segment PCI support, a bus number of 300 (over 255) would immediately tell the admin that slot has no card installed.