We need to consider any gap between what a person can understand and what they can say. Receptive communication is what you can understand. Expressive communication is what you can say. Any gap is an expressive/receptive gap.
To review, receptive communication describes what someone is able to understand. Most of us understand more than we can say. We can show this by following directions or looking at what is being talked about. Expressive communication describes what we can say, or communicate with others. We might express ourselves by speaking. We might also write, or use sign language.
Is there a gap between what a person can say and what that same person can understand? Often, people with complex communication needs often understand more than they can say. Sometimes a lot more.
The bigger the gap, the more important it is that we consider a more robust way to express.