This read-only integer (0..0xffffff) is a bitmap where each set bit represents a DS0 on physical line 7 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 7 to see which DS0 timeslots remain free on that specific line. For example, an administrator provisioning a new port would check portsUsedLine7 to confirm this near-last line still has unclaimed DS0 capacity before assigning DS0s to the new port.