Some credits transfer directly depending on the school and course description, other classes/departments require validation tests even if you already have the college credit. It will allow you to take a much lighter load (the mentioned minimum 15) or if you want to get more out of USAFA, add a language minor or a double major. 15 credits is usually only 5 classes - most semesters, most cadets will have 6, but taking 7 or 8 can be done (not highly recommended though...still don't know what I was thinking)