This OID classifies what kind of address space this PCI memory entry maps into, whether unknown, I/O space, general memory-mapped space, or an expansion ROM. An admin uses this classification to correctly interpret the accompanying base address and size fields, since an I/O-space entry and a memory-mapped entry are handled very differently by the system. For example, a value of exp-rom(4) would tell the admin this particular entry describes the memory range reserved for the card's option ROM rather than its runtime register space.