This read-only integer (0..0xffffff) is a bitmap, indexed by frPortsUsedLineIndex, where each set bit represents a DS0 on that particular DS1 line already claimed by some logical port defined so far; the most significant byte is not meaningful for a DS1. An administrator reads this, for a specific line selected via frPortsUsedLineIndex, to see which DS0 timeslots on that line remain free before creating a new channelized logical port there. For example, retrieving frPortsUsedLine at index 40 shows the DS0 allocation bitmap specifically for physical line 40 on a large card supporting up to 56 lines.