Here is a tentative terminology map.

Domain

a combination of memory caps (ADs) defining an execution environment for the duration of an activation.

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.