In the concept of set, the relationship between elements and sets belongs to (does not belong to), while the relationship between sets is inclusion (also called subset relationship) (does not include).

There are two possibilities for inclusion relations:

(1) equals, and the two groups are equal, indicating that there is an inclusion relationship; (Set contains and equals)

② True inclusion: A really contains B, that is, B is the proper subset of A; (Set contains and is not equal)