BC (Business Component) can be treated as atom in a BPT. It is the basic element to build a test.
A BPT (Business Process Test) can contain many Business Components. It can also contain the flows. It is normally a comprehensive test designed for a particular functional testing scenario (which means a BPT is usually for testing a particular feature of an application).
A Flow can only contain Business Components, but not other Flows. It defines the sequence of the Business Components. It is also kind of test.
The concept of Group is introduced to implement iterate more conveniently. It can contain Business Components or Flows, but not BPT.
To summarize, BTP can contain Group, Flow, and BC. Group can contain Flow and BC. Flow can contain BC.