For example:
http://www.domain.com/scripts/create_user.php
1) Script
2) Event
3) AJAX
4) RPC
Script would fine but this is part of an MVC architecture so that doesn't really jive. Event, is really either or, could return HTML might not. AJAX same thing but without a refresh.
For those reasons I'm leaning towards RPC (although strictly not RPC as the handler will typically be invoked using AJAX locally).
I suppose the process of sending a client request to a remote server is something of RPC, but still.
These are rough definitions I have provided, i'm not looking for indepth disscussion on what each actually are...I just want an opinion.