Subset: For two sets A and B, if any element of set A is an element of set B, we say that set A is contained in set B, or set B contains set A, which means that set A is a subset of set B and an empty set is a subset of any set. Any set is a subset of itself. An empty set is the proper subset of any non-empty set. Complete set: when studying the relationship between sets, these sets are often subsets of a given set. This definite set is called a complete set. Complement set: Generally speaking, let S be a set, A be a subset of S, and the complement set (or complement set) of all elements in S that do not belong to A is called CsA. In other branches of set theory and mathematics, complementary sets have two definitions: relative complementary sets and absolute complementary sets. Complement set can be regarded as the subtraction of two sets, sometimes called difference set.