Great question. An orchestral excerpt is essentially a solo or highly demanding section of an orchestral work with saxophone. To make your life easy, get the book "The Orchestral Saxophonist" volume one by Bruce Ronkin. This book has already extracted the major excerpts for saxophone and you can choose which you want to play. However, since I went to MSM, I'd suggest these if you're auditioning on alto:
Bizet, "L'Arlesienne" (2nd orchestral suite), Mvt. II Intermezzo
Rachmaninoff, "Symphonic Dances"
Mussorgsky/Ravel, "Picture at an Exhibition"
Feel free to PM me if you have any questions about the school, etc.