What does the Union sign in EER diagrams mean?
It means that the publication can be of type chapter, article, or book. That makes publication different from overlap and disjoint. 

When it comes to disjoint or overlap of entities, the topic becomes about a super class and a subclass. 

This, however, is more like a kind of category. 

For example, if an organization has an owner, and if the owner can be a person, a company, or a bank, then it is a union of entities, if they are separate "tables".