This topic adopts the mathematical thinking method of classified discussion. The key to the mathematical idea of classification discussion is the classification standard, which is measured by a ruler (classification standard). This topic is the operational relationship between sets, where B is contained in A, A contains B, B is a subset of A, an empty set is a subset of any set (including itself), and an empty set is a proper subset of any non-empty set (excluding the empty set). These statements are the criteria for classification.