The Endless Plains is the afterlife for cats who are outsiders, such as kittypets, loners and rogues. You can also find lost spirits here, such as those who don't believe in Starclan or the Dark Forest, or ghosts with nowhere to go. Similar to StC and TDF, they keep their wounds from when they were killed, but don't have an aura like void spirits do.
Endless Plains residents don't usually find themselves to be guardians, but it can happen despite being rare. The chances are slightly higher for cats who've been in a clan, but don't believe in afterlife.