This read-only integer (0..0xffffff) is a bitmap where each set bit represents a DS0 on physical line 4 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 4 to see which DS0 timeslots remain free on that specific line. For example, on a card with lines 1-3 fully allocated, an administrator would check portsUsedLine4 next to find remaining free DS0 capacity.