Noun, Suru verb, Transitive verb
proper use; using (different things) for different purposes
引用した問題ではありませんが、関係代名詞と関係副詞の使い分けを問う問題がリーディングセクションにはあります。The problem quoted isn't one, but there are problems in the reading section that ask you to distinguish relative pronouns from relative adverbs.
Also written 使いわけ【つかいわけ】、使分け【つかいわけ】