When commissioning from ROTC into the Air Force, how is it decided, what job you get? is it by the ones you've requested, by need, or others?

At the end of the day it's the needs of the Air Force that will ultimately decide, but you get some say in it.

In your AS400 year (Around October) of ROTC you will get to make a dream sheet of 6 different AFSCs. You'll log into a system called WINGS and update your information to reflect your experience from college. Each class you take in college counts as experience in the system. So you'll go through your transcripts and match classes to the experience guide that your Cadre should give you. This helps the AF to place you in the right AFSC based on what you did in college. The last step is to pick 6 AFSCs in order of desire.

Once all that is done and submitted the Air Force should take your dream sheet and relevant experience and try matching you to an appropriate AFSC. You should find out your assigned AFSC about a month later.

As I said up-front; ultimately the needs of the Air Force will dictate. You might get your #1 choice or you might get an AFSC that's not on your list at all. It all depends on what the Air Force is projecting they will need when you commission.

(1) Needs of the Nation
(2) Your Cadet Rank
(3) What you request, senior year.

-----May the odds, be ever, in your favor----

If you’re qualified to be a pilot, nearly 100% you’ll get it. Otherwise it’s needs of the Air Force and then all of the stuff Maj (Join to see) said. I learned if you are faced with selecting both AFSC and location you are more likely to get the location you want if you take the less desirable AFSC and vice-a-versa.