This read-only integer (0..0xffffff) is a bitmap where each set bit represents a DS0 on physical line 2 that has already been claimed by some logical port defined so far on that DS1; the most significant byte is not meaningful for a DS1. An administrator reads this before creating a new channelized logical port on line 2 to see which DS0 timeslots remain free on that specific line. For example, comparing portsUsedLine2 against portsUsedLine1 lets an administrator see that line 2 has far fewer DS0s already allocated than line 1, making it the better candidate for a new port.