You don't have to fill in anything, they are completely optional. Just select your mod and hit "Convert".
The only reason to use the "Category" is when you have a mod like say HistoricX which throws all it's car into the main directory and you want to group them all nicely put into a HistoricX folder. Same with tracks. Just a way to make it look a little more cleaned up in the GUI, doesn't affect anything else (no idea about multiplayer).
The "Reiza Classes" is a more important, as the class determines which cars you can race against. So if you have say a F1 1995 mod, you have to make sure that all of it's cars are in the same class, otherwise you end up with a field of 20 Ferraris and nothing else. For most mods you don't have to change anything, but for those that cause trouble hit "Clear Vehicle Class" and enter something like Reiza Class: "reiza5,F1-1995". The reiza5 has to be in there as only classes starting with reiza1-reiza10 show up in the game, even in the new GSCExtreme with it's "All Vehicles" tab that seems to still be needed.
"Track Filter Properties" can be used to mark a track as kart track or regular racetrack. It is now mostly obsolete, as GSCExtreme allows you to race on both kart and regular tracks with all cars and karts.
