This read-create integer, ranging from -1 to 2147483647 with a default of -1, is a subinterface identifier used mainly for cross-referencing entries with the device's command-line interface rather than for SNMP-only management. If left unconfigured or set to -1 at creation, the system automatically allocates a nonzero value that can then be read back from this object; a value of zero is reserved and not usable. Any explicit value an admin does supply must be unique among the subinterfaces layered on the same underlying Ethernet interface.