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As far as I understood, it means radiation boundary has to cover the source. For example, create airbox and assign rad. boundary to its faces. Source should be inside the box and this radiation boundary is convex according to the source!
Sorry for the late reply, but thanks for your help. In my case, I have a waveguide slot antenna. I define the radiation so that is adjacent to the slots, but it doesn't enclose the whole waveguide. In this case, the word convex doesn't really make a lot of sense to me. I guess, the radiation box does cover the sources (slot surfaces) of the radiated field, but calling that "convex" does not seem like a good use of the term. Anyway, I get proper results, so I guess that's the thing that matters.