Builds of the full game with some testing/debugging options enabled. Note that official releases (on the Google Play Store, Amazon Appstore, etc.) contain extra features and are likely more stable, so it is best to install from those places when possible if you are purely interested in playing.

For running your own headless game servers.

Sign up to receive test builds on Google Play (Android). This is the full game with Google-Play-Games logins, in-app-purchases, etc. Testers may receive development versions of the game not ready for the general public and also receive final release builds before the general public does.