This read-only bitmap tracks which DS0 timeslots on physical line 6 have already been assigned to existing logical ports, with the most significant byte invalid for a DS1. An administrator consults this before creating a new port specifically on line 6, since the DS0 usage on other lines has no bearing on what's available here. A fully set bitmap for line 6 means an administrator must either free existing ports there or provision new capacity on a different line.