This read-only integer (1..56) is the index into the table that holds the per-line DS0 usage bitmap (frPortsUsedLine) across as many as 56 DS1 lines, generalizing the fixed portsUsedLine1-through-8 objects to a larger, indexable line count. An administrator uses this index to select which of up to 56 physical DS1 lines' DS0 usage bitmap they want to read from the corresponding table row. For example, on a card with more than 8 physical lines, such as an FRSM-2CT3, frPortsUsedLineIndex lets an administrator retrieve the DS0 usage bitmap for, say, line 40, which the fixed portsUsedLine1-8 objects could never address.