This read-only integer (0..0xffffff) is a bitmap where each set bit represents a DS0 on physical line 6 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 6 to see which DS0 timeslots remain free on that specific line. For example, on an FRSM-8T1/E1 card with 8 physical lines, portsUsedLine6 lets an administrator check the sixth line's DS0 allocation independently of the other seven lines.