Receptive communication skills are the information that is going into his head (so to speak), and how he is processing it. If he has attention problems, it is not unusual to see that they might be expressive language (communication) problems. They sometimes go hand in hand. If he can't pay attention, he's not going to get the information he needs. If he doesn't understand what is going on, his attention will go elsewhere.
As another said, sometimes what you see as a parent isn't always as objective as what an outsider sees. That doesn't mean you didn't answer properly, just from the perspective of a parent.
Is your child a bit "spacy", where he just spaces out on things when talking? How do you have to give him directions? Do you make him repeat them? Does he have problems with two and three step directions? If you asked him to do two or three or four things in succession, would he remember them? Those are the things you have to think about.
I don't think they mean to give him a behavior test to determine if he's a problem. It is more to see where his difficulties come from. It seems the school is using this as a diagnostic tool to find out where his strengths and weakness lie to help remediate them. Have you asked the person who gave him the test to explain everything to you thoroughly? You do have that right to know. You shouldn't be made to feel foolish because you don't know or understand.
If you go to the website for the test, it lists this table to explain the test elements:
Content Description of Vineland-II Teacher Rating Form
Domains & Subdomains Content
"Communication How an individual speaks, understands others, and uses written language "
"Receptive How the student listens and pays attention, and what he or she understands "
Understand that if there are receptive problems, that is different from attention problems. Sometimes these go hand in hand, but receptive language skills can be remediated through a speech therapist. If they are saying he has difficulty in this area, you should ask for a speech therapist to test him for receptive language difficulties.
ADD/ADHD can only be diagnosed by a medical doctor, because if there is medicine involved, it has to be prescribed by a doctor. While a regular pediatriatian can do those things, most times a neurologist or psychriast specializing in ADD/ADHD is the better choice (IMHO) for diagnosing that condition. You can go to some websites to look for a doctor, or even your health insurance carrier. The school may have one that they use to for diagnosing ADD, find out if there are other parents who have taken their children to him/her and what their impression is.
I would ask why he isn't finishing his work. What is making him get off task? That is what this test, I am assuming is trying to determine.
Good luck