Beta test realm (called by the acronym "BTR") is the unofficial name for the test realm (or realms) used for Beta phase testing. So far there are no official info or details about this realm or realms. In theory, it functions similarly the test realm (or Alpha test realm).
Battle for Azeroth
This section concerns content exclusive to Battle for Azeroth.
BTR realms were the same as the ATR realms.