From what I understand you can choose your campus until that campus is filled. After that, you go where they have a spot. I think that for the most part it is first come first served (and you have to submit your deposit to hold the spot you have requested).
I'm not totally sure but this is what I got when I went for my interview:
The first 120 people accepted get to choose which campus. For the 40 people after that, when you call you can choose whichever campus, if there is still spots left. That means if all 120 people who got accepted chooses Baltimore, as the 121st person, you will be go to Shady Grove. But this possiblity is small, since I'm sure there are people who prefer Shady Grove.
Not to discourage anyone, but UofM isn't on rolling admissions. They sent all the acceptance emails by email on the same day. I was told during my interview that they would not send out rejections by email, but I've heard a few cases where that's not the case.