I would probably define it as a photographic process that is not currently done by the masses.
I know that sounds vague, but let me offer a few examples...
When Palladio was offering PT/PD paper, it was commercially made. It was still under the AP umbrella.
Some would say Polaroid transfers and SX-70 manipulations fall under the AP category as well. While it is a commercially made product, it is a manipulation of materials.
Photocopies and transfering of images... There are a bunch of different ideas out there.
Pt/Pd for example was once considered a normal way to make an image... Not an alternative process.... Just like the fiber paper and film I'm using today... One day it too will be considered an AP.