This is the IP address of one specific hop (a router interface the path must or should traverse) within an explicit tunnel path, and the MIB notes it cannot be changed once that hop entry has been created -- to alter it, the hop must be deleted and re-added. This immutability matters operationally because fixing a typo'd hop address requires a delete/recreate cycle rather than a simple modification, which could briefly affect any tunnels actively using that path. If a tunnel keeps failing to signal along an expected route, verifying each hop's IP address here against the intended physical topology is a direct way to catch a misconfigured waypoint.