A System Reference Document (SRD) is a description of a role-playing game's mechanics licensed under the Open Game License 1.0a (OGL) to allow other publishers to make material compatible with that game.
Occasionally, SRD is used to represent System Resource Document.
A "system reference document" or equivalent does not exist for Powered by the Apocalypse. Games using the system (there are dozens) are noted as "Powered by the Apocalypse" (PbtA) or as using "Apocalypse World Engine". This is allowed (and encouraged) by D. Vincent Baker who authored the original system. The only "authoritative" statement regarding the system's use can be found in "An open letter re: Powered by the Apocalypse" at http://apocalypse-world.com/pbta/policy
Users interested in seeing the system devoid (more or less) of setting should look at Simple World as an instructive case.