This describes how the adjacency is used: on a point-to-point link this might be level1and2, but on a LAN, usage is level1 on the adjacency between peers at L1 and level2 for the adjacency between peers at L2. It highlights a subtle IS-IS behavior difference between media types -- a point-to-point link between two L1L2 routers forms a single adjacency used for both levels at once, while the same two routers on a shared LAN form two entirely separate adjacency entries -- so an admin counting expected adjacencies must know which media type is in play or miscount by a factor of two. For example, two L1L2 routers connected by both a point-to-point serial link and a shared LAN segment would show one combined level1and2 adjacency over the serial link but two distinct level1 and level2 adjacencies over the LAN.