You send a deposit/acceptance to all schools that you may matriculate. You can hold multiple acceptances until May 2, although this deadline is in mid-April for schools that begin med school earlier in the fall.
The schools want to pressure you to either commit or not commit to them, because every year at most schools a large proportion of their accepted class chooses not to attend. It is protocol to hold multiple acceptances before May 2, and it is courtesy to let a school know that you will not attend as early as you know you are going somewhere else so that they have more time to fill their class.
You are 'accepting' their offer of matriculation. You are not committing to matriculate, although you are showing interest by accepting their offer. Some students decline acceptances immediately, and other students hold on to acceptances toward the deadline when they know where they will go. The school can only have a certain number of people who accept their offer of matriculation at one time, or else their class may be too large.