zip code and the program finds the closest location for you. And if you already have an appointment and need to change it, the new service will cancel your current appointment and reschedule it.
The advantage of this is that for those out there who are not thrilled at the prospect of spending time in an AT&T store waiting on their turn (who could get bored looking at all of those new phones?), with a specific appointment, you will know exactly when you need to be at the store saving you all of that waiting time. As for us, well we don't mind checking out all of the phones, but that is our job, after all.
source: AT&T via Slashgear