Notes on the Biin Specs:
Here is a tentative terminology map.
The description of ports is in section 4.16.2. The send command is described in section 18.125. The calld instruction seems to be the gate invocation on page 18-23. Domain is discussed at 2-8. Domain call is at 4-12.
I cannot find a description of the shape of a message. I don’t know whether a message can be sent across a protection boundary.