Realm of Possibility

"The universe outside Continuity is a place where anything can happen. It's the same as the real universe, but the Laws of Physics and Time don't necessarily apply here..." -Narrator Daniel, Azumanga Who

The Realm of Possibility (or ROP) is a "parallel universe" to the Realm of Continuity (or ROC) which features unlimited Possibilities – the only limit being the imaginations of those who reside therein. In comparison, the ROC stays on a "track" of sorts.

The Realms have only appeared in one of Daniel's stories: namely, Azumanga Who.

Summary
The anime and manga of Azumanga Daioh is the "center" of the ROC, although this idea can easily apply to any fandom (Harry Potter, House M.D., Shakespearean plays, etc.). As each author has different preferences, stories in the fan community may be placed in either Realm as the author decides.

For example, a story that tells an alternate take on the "core events" may be ROP (being an alternate take) or ROC (fleshing-out established events), depending on what elements are changed. Stories that would likely be automatic-ROP include those which drastically change the core events, such as a major character dying.

Planes of Reality?
In late December 2011, the author had a dream in which the ROP sometimes resembled Moonside (from EarthBound). Part of the dream involved what appeared to be ROC Daniel in the ROP, and talking to a woman who said an associate "went up" to Continuity.

Another part involved ROP Daniel getting in the car of an officer who somehow found himself there. While Daniel's home remained in the Moonside colors, going outside he expressed shock at the fact that everything looked normal. It was while driving down that he realized the officer had touched Daniel's machine which could screw with things to an extreme.

This would suggest that Continuity and Possibility are in fact planes of reality.