The whole playlist. You have saved that particular random song earlier individually.
There are two scenarios for offline saving:
1? Saving an entire playlist, album or artist will leave an iPad or iPhone sign on only the album cover up top
2? Saving an individual song will leave an iPad/iPhone sign on the cover of that particular song.
Now, if that individual song is part of an offline album/playlist it will keep that sign.
If you go to Settings > General > Storage & iCloud Usage > Music, you will see which songs are saved on your iDevice. Also, if you choose to show "Music Available Offline" you will see which songs are available offline as well.