[ImpactBreaker]

Sometimes a-sides won't make it to an album: "LOVE~Destiny~" from ayumi hamasaki, "the SPEED STAR" + "Did U" from namie amuro, and "Under The Moon" from Do AS Infinity are all a-sides that didn't make it for an album. (later on "LOVE~Destiny~" made it to A BEST and "Under The Moon" made it for "Do The A-Side", proving they indeed are a-side songs even though the first didn't make it for LOVEppears and the later didn't make it to "TRUE SONG")

The concept is indeed subjective, but some things can be concluded by default:

-If the song made it to an album, it is an a-side, unless the album has a bonus extra, which could be considered either an a-side or b-side
-A song that doesn't make it to an album isn't necessarily a b-side
-Extremely experimental, bonus tracks and cover songs are usually, but not always, b-sides. That's because b-side was the other side of single vynils (in the past when there were no CDs) that usually carried extra songs like performances, interviews and bonuses, while the main tracks would be on the a-side of the vynil. If you have listened to a few old single vynils you'll know what that concept means.