The inclusion symbol is used to indicate that one set is the proper subset of another set. If A is really contained in B, it means that the set A is really contained in the set B, or A is the proper subset of B. & amp#8843; Or&# 8834 & # 8835; (two ways of writing),&# 8842; (really included in)&; #8843; (true tolerance)

Proper subset:

If set A is a subset of set B, and at least one element in set B does not belong to a, then set A is called the proper subset of set B. If A is contained in B and A is not equal to B, then set A is called the proper subset of set B. ..

If a&; #8838; B, there is an element X ∈ B. Element X does not belong to set A. It is said that set A and set B have a true inclusion relationship. Set A is the proper subset of set B, and it is recorded as A &；; #8842; B (or b&; #8843; A), read "A does contain B" (or "B does contain A")