This provides the ifIndex of an encapsulation layer higher up a port stack when the IP interface's true binding isn't directly represented in the standard Interfaces MIB but a related layer above it is; it's set to zero whenever the primary binding object already has a nonzero value, since the two are mutually exclusive ways of locating the interface. Administrators use it as a fallback lookup when the primary binding index is zero, to still trace an IP interface back to some meaningful entry in the interfaces table. If the primary binding object reads 0 but this object reads 7, an admin would look up ifIndex 7 instead to find the relevant port-stack layer.