What is Principal Acting?

Question: What is Principal Acting!?

I was reading this Disney Channel auditions site and it came up with the address to send the audition to and in one of the sections it came up with
Principal Actors casting
what is it!?Www@QuestionHome@Com

Best Answer - Chosen by Asker:

I'm pretty sure that it means that its only for professionals!. It may be a movie or show or an acting agency though!. But I think it has to do with actors that already are famous!.Www@QuestionHome@Com