usrInverseARPIfIndex identifies, using the same numbering as the standard ifIndex, which Frame Relay interface a row of the Inverse ARP cache applies to, per its description. An admin uses this, together with usrInverseARPNetAddress since both form the table's INDEX, to know exactly which physical or logical interface learned or holds a given DLCI-to-IP mapping. For example, a usrInverseARPIfIndex value of 5 ties that cache row's DLCI or address mapping to interface 5, the same interface number reported elsewhere via standard ifIndex.