When the client wants to join a match on a dedicated server hosted on playfab by multiplayer servers 2.0 it needs to supply latencies for regions so matchmaking can select the region with the best ping. Currently the client cannot know what regions a build is deployed to. So it has to have some list of regions compiled in so it can ping them. But if a new region is added the client has to be changed/redistributed.
It would theoretically be possible to allow the client to call the serer API but this would give the client more control than desired (it could start it's own servers!).
Currently the best option seems to be to write cloud script to allow the client to make a query of the regions of the build a matchmaking queue is using.
Since the region selection rule is mandatory when using multiplayer servers 2.0 with automatic allocation from matchmaking it would make a lot of sense to add a query to the matchmaking client API that gets the list of regions a build connected to a matchmaking queue is deployed to so the client can ping the right regions.