Full Idea
X ∪ Y indicates the 'union' of sets X and Y, which is a set containing just those things that are in X or Y (or both).
Gist of Idea
X∪Y indicates the 'union' of all the things in sets X and Y
Source
Graham Priest (Intro to Non-Classical Logic (1st ed) [2001], 0.1.8)
Book Reference
Priest,Graham: 'Introduction to Non-Classical Logic' [CUP 2001], p.-6