First Deposit Deadlines?

I understand that the first deposit is non-refundable and is used towards your first semester tuition. Not paying the first deposit means you forfeit your seat in the incoming class. BUT, does paying the first deposit mean you are firmly accepting the school? Or, can you pay the first deposit to multiple schools (because you are undecided, or still waiting to hear from waitlist schools, etc...)? When do deposits become firm acceptances, in which case all other applications must be withdrawn?

What happens if you get off the waitlist in late summer (e.g. August), but you've already firmly accepted a school (because you would have had to to not lose your seat in the incoming class)?