It means that 'Books', 'Chapters' and 'Articles' are sub-classes of the super-class 'Publication'.
'Books', 'Chapters' and 'Articles' are categories of 'Publication'
You can check out this EER reference for more details and what not.
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".