This identifies which specific interface on the source chassis (identified by circuitSrcIpAddr) this circuit originates from, since a chassis can have multiple physical or logical interfaces any of which could be the actual circuit endpoint. An admin needs to set this correctly alongside the source IP address, because getting the wrong interface number would mean the circuit is provisioned against the wrong physical port even though the chassis address is correct. For instance, if the source chassis has interfaces numbered 1 through 8, setting this to 3 ties this circuit specifically to the third interface's fiber or copper connection, not any of the others.