Nero Digital Subtitles


I’m working on a master thesis about how to get text in an MP4 file.
One of the possibilities is using Nero Recode 2 to get the vobsubs from a dvd directly into mp4 (nero digital).

I’m looking for some “specs” about the Nero Digital Vobsubs.
I dont think i will be able to find a real spec from nero, but maybe someone can point me to some document with more info or give me some usefull information about the format nero is using.