This read-only bitmap shows which DS0 timeslots on physical line 1 have already been claimed by logical ports defined so far on this card, with each set bit marking a used DS0 and the most significant byte left invalid since a DS1 only has 24 timeslots. An administrator checks this before creating a new logical port on line 1 to see which DS0s remain free rather than risking an overlapping timeslot assignment. For example, if the low byte of cesPortsUsedLine1 reads all 1's, all 8 DS0s in that byte's range on line 1 are already assigned to existing ports.