Appearance
Playlists
Library → Playlists, and Spotify playlists separately.
Your Plex playlists
Playlists from Plex, playable end to end. Opening one gives you the tracklist; pressing play queues the whole thing, with shuffle and repeat available.
Spotify playlists
Kept apart from Plex playlists deliberately, because they behave differently: a Spotify playlist can reference tracks you do not own. Media shows the whole list and marks what is missing rather than quietly dropping it — the gap is usually the interesting part, since it is a ready-made list of things to find.
Generated playlists
The playlist generator builds a list from what you ask for and queues it directly. Unlike radio, which is endless and adapts as you listen, a generated playlist is a fixed set you can see in full before playing.