The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
And since FAF is an open source project, anyone who feels inclined to go read the code for themselves can also do that here:
https://github.com/FAForever/server/blob/6c2200c93fa6fc677fbfd3a114afce2e47a45771/server/matchmaker/map_pool.py#L24
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