Higher operating pressures, lower weight standard bullet (200gn vs 230gn), rim the size of a .40 S&W's, designed to capitalize on the magic "45" number.
As far as I'm concerned, there's no advantage over the 9mm - and it was designed around fitting into a gun that size.
To answer the OPs question, on Gallery of guns, they simply abbreviate "ACP" as "AP." They are the same. I assume the AP means auto pistol.
It's got nothing to do with the .45 GAP, a round which is a testiment to Gaston's considerable ego, and a reality check as to just how far into the shooting world a household appliance maker should delve.