Can there not be situations where neither code is strictly narrower than the other but instead the conceptual ven diagrams overlap. That's a stronger statement than 'related-to' which means they could be equivalent or narrower or broader. If you know definitively that the concepts aren't equivalent, narrower or broader but do have overlap, there should be a code to say that. This may not happen often, but without the concept, the conceptual space isn't covered, which makes a 'required' binding problematic.

(Comment 110 - imported by: Ron G. Parker)