What does booking an aptitude index mean?

I booked a general aptitude index (90 asvab 95 G score) and I can’t for the love of God figure out exactly what it means. I see some people saying there’s jobs set aside specifically for that and I see some saying that I just choose from a list of jobs that are left over. How does it work? What’s the likeliness of me getting forced into services? Can I see what jobs I’ll be choose from before I go?