Unlockable stage

An unlockable stage is a stage that is not available at the start of the game (unlike a starter stage), but must be unlocked. Just like an unlockable character, is required to accomplish a particular in-game feat and once complete, a message will tell him the stage is unlocked.

Super Smash Flash 2
Since the game is still under development, any stage hasn't been confirmed for to be an unlockable. Some where mark as unlockable but aren't 100% official.