Uniquely requiring the two elements... notsomuch. Uniquely _including_ them, yes.

I think it's about finding the smallest grouping/category that also includes both. But... this is at an _absurdly_ non-verbal level for me. So I don't know. And I don't think it's a riddle, or not meant to be one.

Interestingly, I _did_ find my answer to be the only way I could answer. Indeed, I took a couple seconds before answering to verify that I hadn't missed something that felt more 'right'. So, for me at least, my answer is the only one that I could have given.