The Encore levels, unlocked after beating the game (or 7-4 for now), remix rooms from earlier levels with much trickier enemy placements and some new gameplay twists.
Basically, imagine if ULTRAKILL had an additional difficulty mode that changed enemy encounters, but with all the filler cut out.
0-E, while not done yet, will be in the next update. No idea about the rest yet, or how many there will be.