Playlist syncing mirrors your Spotify playlist onto YouTube, SoundCloud, Apple Music, Deezer, or Tidal. The custom track tool lets you hold a song in one of those mirror playlists even when it isn't in your Spotify playlist. Handy for a track that never made it to Spotify, or something you only want on your YouTube or SoundCloud version.
How to add one
What happens then
The track stays put through every sync. Normally a sync removes anything that isn't in your Spotify source; custom tracks are the exception. If you set a removal timer, the song comes off automatically when it's up. To review or remove custom tracks later, open the same "Add a custom track" menu, it lists everything you've already added.