This read-only integer, fixed at value 1, identifies the port number on the OC-3 card, reflecting that each OC-3 card in this platform has a single optical port. It tells an administrator there is no ambiguity about which port a given OC-3 row refers to, unlike the multi-port Ethernet card table. For example, a script iterating over OC-3 port entries can assume exactly one port per card slot rather than needing to check for up to four ports as with Ethernet cards.