My understanding is that there is not a real difference between the actual terms of alternate or waitlist. I'm sure that the verbiage on the on-line portal for people that were waitlisted during the cycle is identical to that for those that received the call from Dr. Arias.
The difference is more in how each individual was added to the list. Those that received the call from Dr. Arias were actually accepted by the adcom - there just aren't seats yet available to give out. As soon as those seats are available (prior to May 15, even), a spot will be offered to those individuals without further review of their files (the decision to accept has already been made).
Those that received an email to inform them that they have been added to the waitlist/alternate list (w/out a call from Dr. Arias) were given that status, rather than an acceptance, by the adcom. Therefore, no formal decision on whether or not each of those individual will be accepted, at the time that seats open up, has been made. Since they don't take updates until after May 15, I'm guessing that they don't start to make these decisions until that time.
I could be wrong, but that seems to be the situation to me.