See https://github.com/facebook/phabricator/issues/596
Broadly, Facebook would like to bring Pull Requests from GitHub into Phabricator.
In the long term we can do this properly via Doorkeeper/Nuance, but that's probably a ways off. This seems like a reasonable low-budget compromise for now.
I'm a little hesitant to add a ton of parameters to this call, but repositoryPHID seems pretty reasonable, and is notable because it also controls default policies.