Finding a song on a compilation can hide the reason people call it a B-side. There may be no image of the original single, and a streaming listing can put a reissue date beside music recorded decades earlier. Before treating that date as the song's birthday, look for the release it came from.
“B-side” began as a place on a record. It remains useful as a piece of release history, even when there's no disc to flip. It isn't a rating. The label alone won't tell you whether the track is an unfinished outtake, a different mix or the same recording you've saved elsewhere.
What Rain's first release tells you
The Beatles' official Rain page calls it the B-side of Paperback Writer. Both songs were recorded during the Revolver sessions; neither was on Revolver. The Paperback Writer entry gives two release dates for the single: May 30, 1966 in the US and June 10 in the UK.
There's a useful distinction tucked into that example. Rain belongs to the history of those recording sessions, but an original-album track list won't take you to it. Listening through Revolver and following the related single are two different ways into that period. You need the single if your question is what else the band recorded around the album.
MusicBrainz's release-type documentation explains the physical arrangement: a vinyl single's main song on the A-side, with another on the reverse. It also allows for singles containing several tracks or versions. Calling something a single in a digital library therefore doesn't require a one-track release, any more than calling Rain a B-side requires you to own the vinyl.
The same song on another release
A compilation provides another route to music that sat outside the albums. The Beatles' Past Masters page describes 33 non-album tracks, originally collected on two CDs in 1988. Its purpose was to gather material that an album-only collection could miss. The compilation's date describes that collection; it doesn't make every song on it a 1988 recording.
The labels you meet along the way do different jobs. A B-side was part of a single. A bonus track is an addition to a particular edition. “Compilation track” describes the collection in front of you. One song can acquire more than one of those descriptions as it moves between releases.
That still leaves the question of the actual audio. MusicBrainz's recording model lets one recording connect to tracks on several releases. It treats a live performance or a remix as different recorded audio, while mastering alone doesn't create a new recording in its model. These are the database's distinctions, which help explain the credits; they're no guarantee that every streaming listing has been entered correctly.
A useful note for your library
Instead of saving “rare version,” write down the original release role, the edition you're playing and the version information given in its credits. For Rain, the first field can say “B-side of Paperback Writer.” The second should name the release you've actually opened. Use the third for a stated mix or performance description, if the source supplies one.
Don't fill that last field by guessing. A matching title isn't an audio comparison, and a different date isn't proof of a different performance. Our remaster and remix guide covers those production terms; the album-credits guide shows where to look for the people and edition details.
For a listening session, start with the two songs on the single. Then return to the album recorded around them. You'll be following a release relationship you can explain, without asking the B-side label to decide which song deserves your attention.