I've mostly seen this term used in the "stamp" world.....a company will state that they are an "Angel Stamp Company."
Then they will state the policy....It could read something like this:

We Allow individuals to use out stamps items for resale as long as ALL RESALE ITEMS using our images are hand stamped without any form of mechanical reproduction of the stamp image. No further permission is required for this specified use.
The policy has to do with the use of their product and there are different "angel policies."