Delete empty

"Empty" means the collection has been built at least once, its preview resolved, and none of its titles are in the library Sonarr or Radarr feeds. Not-yet-built collections are excluded (they were never evaluated), and so are collections whose preview errored or is still loading — those also show 0 items, but their emptiness is unknown, and sweeping them would delete a populated collection on a transient failure.

This button deletes the collection from Kustos as well as Plex, permanently. Its idea of empty is "nothing in the arr-fed library", not what Plex holds. If no built collection resolves a single title, which usually means Sonarr or Radarr does not feed that library, the button is turned off and says so. Use it only for collections you never want back.

The automatic sweep is different. While delete below minimum is on (it is by default; Settings → Media management), an empty collection is only taken out of Plex. Its definition stays in Kustos, and it comes back once it has members again. Seasonal collections outside their window are left alone by both.

Orphan shells are a separate button. They have no Kustos row, so deleting them touches Plex only — including collections Kustos never created. Plex keeps a collection after its last title is deleted; those hold 0 items and aren't managed by Kustos.