This is a specifier for the part of the interface ifIndex to which this circuit corresponds, such as a DLCI or VPI/VCI, and it cannot be modified after creation. It lets IS-IS run over a sub-channel of a shared physical interface, disambiguating multiple independent IS-IS circuits that would otherwise all point to the identical ifIndex. For example, two IS-IS circuits sharing the same physical Frame Relay interface's ifIndex would use different sub-index values, such as DLCI 100 and DLCI 200, to represent two logically separate point-to-point adjacencies over that one physical port.