A={ 1, 2,3} and b = {4,5}, then A∪B={ 1, 2,3,4,5}, which is a set of elements belonging to both A and B.

A={ 1, 2,3} and B={2}, then A∪B={ 1, 2,3} is the set of all elements belonging to A.

A={2} and B = {2,4,5}, then A ∪ B = {2,4,5} is the set of all elements belonging to B.