Someone picked it at one point back in the Air Warrior days (maybe DoK), and it sort of stuck. We just needed a term for the different pieces of a scenario. "Battle" didn't seem to fit, as the scenario iteself is geared to be one battle, so it would be confusing if we referred to there being multiple battles. We used "part" already to describe scenarios that were one big scenario, broken up into two or more smaller scenarios (Longbow, part 1; Longbow, part 2; etc.). "Days" doesn't work as people confuse that with regular days.