I asked about this..apparently, it is to do with the A side being the side most representative of the label. For example, the Acid filled wired sound that is Stay upForever will usually be on the A side. Then the AA/B will be a more experimental (pushing the boundaries of sound ) version that is there to dip one's toes in the water.

Having said this, I bought a techno recorda record that had E side and K side. Now I haven't got a clue what ths is meant to mean...