The draft registration cards on earlier calls did have application information for joining specific jobs. If you were a technical professional, or had some sort of training that could prove your proficiency in a certain area, then you could work with administration to go into your desired MOS.
As my grandfather showed the case it was a simple situation. "People are stupid and crazy." A lot of young men simply either rushed in, or hung their heads while going into GI without even trying to find out what they could do.
Continuing through the war as the Allied forces needed more people, it became harder and harder to get into certain rates. The Navy also became pretty much locked up.
As for medical reasons, there were a lot of people who faked their problems to avoid the draft. No getting out of that. The government examines you.
