I am a professional musician with a relatively extensive background in music. I also play many different genres of music with people who don't necessarily understand theory. For example, sometimes people will say "play a 2 chord" (we are in the key of G and they want an A chord to be played, which is really a V/v). I understand this, but they don't. I find this one easy to explain so that they know that an A chord is a V/V, and a ii chord would be Am.
What I struggle with is when there are unusual chords and they say play a 3 chord and they want a B7 not a B minor. How do I explain the B7 in a way to make it less confusing to all? ( I was recently in a jam session this past weekend and there was a very long, argument by one member about it. I didn't get involved at all because, I can't give a logical number expanation for that chord in the key of G). What would be the theory explanation for a B7 in the key of G? I could just say V/vi but I don't think that is correct theory, though it would make it less confusing to explain it that way, I think.