This read-only bitmap tracks which DS0 timeslots on physical line 5 have been consumed by logical ports defined on this card so far, with the most significant byte invalid for a DS1. An administrator managing an 8-line CESM-8T1/E1 card would check this specific line's bitmap when planning where to add capacity on line 5 rather than another line. Comparing this bitmap against cesPortsUsedLine1 through cesPortsUsedLine4 shows which of the card's lines still have unused DS0 capacity.