This read-only integer (0..0xffffff) is a bitmap where each set bit represents a DS0 on physical line 3 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 3 to see which DS0 timeslots remain free on that specific line. For example, a fully set portsUsedLine3 value (all relevant bits on) tells an administrator line 3 has no remaining free DS0s for a new channelized port.