Most terms used in Beaker documentation should be common. The following documents project jargon which may otherwise be confusing.
The Coordinator is the system on which Beaker itself is run. In many environments this will be a local host, typically a developer's primary machine. Used instead of Master to avoid confusion (see Roles).
A System Under Test (SUT) is one of the systems which is the subject of testing with Beaker. Contrast the Beaker Coordinator.