B ∩ D ≠ φ has four meanings: 1. B is the proper subset of D, which means that all elements in B can be found in D, but at least one element in D is not in B.

2.D is the proper subset of B, which means that all elements in D can be found in B, but at least one element in B is not in D, and D is not an empty set.

3.B=D

B and D are not subsets of each other, but they have common elements.

An empty set means "empty" and has no elements. An empty set is a subset of any set, that is, it is universal. When it comes to subsets, you have to think of.