I think that I have worked out what to do. In each of my sets (including sets 7 & 8), I have two answers with something in common. In all bar sets 1, 2 & 6, the other pairs also have something specific (but different) in common. Including those three sets, what those pairs each have in common is more general.
I can't see that my grid can be wrong, given how the numbers and message work, but I'd appreciate confirmation of my description.
I hope that the above gives nothing away to those who haven't solved this puzzle!