This read-create value specifies the sub-interface number the BUS resides on, which may be shared with a LEC or BUS of the same emulated LAN but isn't required to be, chosen by the agent if unspecified, and locked after row creation. Because it can optionally share a sub-interface with a LEC serving the same emulated LAN, an admin troubleshooting resource contention on one sub-interface needs to check whether both a LEC and BUS for the same ELAN are stacked there together. Two different ELANs sharing the same physical ATM port would typically each get the BUS assigned to its own distinct sub-interface number so their broadcast traffic stays logically separated.