Well there's a section where you can indicate which ones you would DEFINITELY be able to attend. And you can mark YES/NO for each section. I guess if you get accepted, they pick a session out of the ones you marked YES for.
I marked sessions A and C. I would rather go to session C, because I would be out of school by then! Session A would be the week before finals and Session B would be the week of finals for me, and I definitely can not miss finals/friends' graduation! I marked session A anyway, since I could work something out with my school if I happen to get session A of NASS. I'd rather get Session A of AFA SS over Session A of NASS since it falls two weeks before finals and the week of my birthday!!