What's a higher position in ballet?

Your rank in the ballet has nothing to do with how you are cast. A corps de ballet member could have a lead in a ballet (like Peter Martins did when he restaged Romeo and Juliet for NYCB) and a Principal can have a solo. The Principal still holds a higher position in the ballet company than the corps de ballet member. Ballet companies have alternating casts and don't have "understudies". If a dancer gets injured, another dancer who performs the part on alternate nights steps in.

* If this is for an amateur production where they may be understudies, you may not get to dance at all as the "understudy." So being a corps de ballet is much better. At least you get to perform.

*EDIT: An apprentice with the ballet would perform and not "understudy" any parts. They just don't get paid and have no contract.. They get their shoes paid for and they get to perform. After a certain number of performances within about a 6 months period, they are either made a member of the company with a contract, or let go.

Core and Background in my opinion because under study is just like an "apprenticeship" or Internship you're not really doing much in the Under Study