It's the order in which the difficulty tutorials are. The first tutorial you'll do will be Arcade, the second Realistic, and the third Sim. You can identify them by the star when you completed them: Arcade is a small bronze star, Realistic a medium silver star and Sim a big gold star.
Realistic is half-way Sim and Arcade: it has name indicators, boxes around the airplanes, but realistic flight models and no lead indicator (for aiming). Simulator is the hardest, with no names, no boxes, just visual identification, and realistic flight models. So in order of difficulty, easy to hard[list]
- Arcade
- Realistic
- Sim