This OID is the index of a specific memory range entry within a PCI function's set of memory resources, since a function can expose up to several separate memory ranges. An admin enumerates entries by this index when a function has multiple memory-mapped regions, such as separate ranges for registers and for a frame buffer. For example, memory index 0 might describe a controller's register space while memory index 1 describes its I/O expansion ROM space on the same function.