Graded drivers are those that have won various championships or are professionals in the top categories. i.e They could be a full time pro in V8SC like Lowndes and Luff, or full time works people like the 2 German drivers in the other Audi. Or they could be "technically"unpaid amateurs" but have won something like the GT championship or the Carrera Cup etc. Also tends to cover some of the blokes who have retired from professional racing but have a lot of experience, like John Bowe of Craig Baird.