Hi. Magicdata is the file "inside" a track that contains the specifics for the track, some only for AI (as setup, fuel load and consumption, % of errors...) and some for both AI and human player (as air resistance, track grip, track banking and bump...): editing it you can create a very personal track.
You can get it from custom tracks with the utility Cmagic, open and edit it with notepad, then riload it into tracks with Cmagic (you can get Cmagic in this site in the old download section\utilities).
You can get the magic data of the original Gp3 tracks with Cmagic but from the Gp3.exe.
I know that Roberto Scandura has made a very detailed guide of the MD file (sorry if i'm forgetting anyone else's job), you can get it on his site or in the download section of this site (i don't remember in which section, sorry).
Remember well: the way of editing the MD is very hard (it takes a lot of time, you must try and try and try to understand well what affects what...) but very satisfaiting (you can really customize the track as you like).
Claudio.