The framework has been built to enable messages (which are expected to be interpreted as commands of some sort) to be sent from one module to another, possibly cross-language. The syntax is as similar as possible in both SystemC and GreenScript(Python).
Typically one of the models would be the test bench, and the other the device under test. Model to Model communication between two device models is discouraged.
GreenMessage provides an easy to use low-level framework on which more complex command and control messages can be sent.