This read-only integer (0..0xffffff) is a bitmap where each set bit represents a DS0 on physical line 5 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 5 to see which DS0 timeslots remain free on that specific line. For example, on an FRSM-8T1/E1 card, an administrator scanning across portsUsedLine1 through portsUsedLine8 would check portsUsedLine5's bitmap specifically to assess that fifth line's available DS0 capacity.