cmIDSLBoardIndex is a read-only Permanent Interface Identifier that encodes the shelf, slot, and port location of an IDSL line card, doubling as its ifIndex. An administrator uses this value to correlate IDSL-specific timing objects like cmIDSLBoardTimingMode back to the exact physical card location in the chassis. For example, an operator troubleshooting a timing alarm would use cmIDSLBoardIndex to identify which shelf and slot houses the affected IDSL card before physically inspecting it.