This read-only enumerated value indicates whether this adjacency is used for Level 1 traffic only, Level 2 traffic only, or both, noting that two separate adjacencies of type level1 and level2 can exist simultaneously between the same pair of systems. It tells an administrator precisely what kind of traffic will actually be routed over a given neighbor relationship, which matters when troubleshooting why only intra-area or only backbone routes are missing. For example, finding csIsisISAdjUsage set to level1(2) on an adjacency explains why that neighbor never appears in Level 2 backbone route calculations.