The FAA defines this. Category is for example: Airplane; Rotorcraft; ummm lighter than air etc. Class would be for example single engine land, multi engine land, single and Multi sea. These can be further subdivided into turbine powered, turbojet recip, and "large" aircraft with a max gross weight of 12,500# or greater.
I think we could come up with several RTW represenative categories. The idea having some real puddle jumper flights, perhaps some rotor wing (mebbe a penalty for reduced realism or crashes turned off) some tactical jets. The idea of having some appropriate challenges for each type. A minimul altitude? reach say 40,000. on the leg for tactical jets.
Ideas anyhow.