How a category differ from regular shared subclass in dbms?

Category and shared subclass both are different terms. Their concepts are separate fields from each other.

The category contains two or more superclasses which may represent different-different entity types, while other regular shared subclasses always have a single superclass.

An entity that is a member of the shared subclass must exist in all superclasses, but in the case of category, a member entity can be a part of any one of the superclasses. Moreover, attribute inheritance works selectively in the case of categories.

A category is used when we need arises for modeling a single superclass/subclass relationship with more than one superclass, where the superclasses represent different entity types.

And the subclass will represent a combination of objects that is a subset of different entity types. For example, a piece of land can be owned by a person, a company, or a bank. All of these entities are different types, but they will jointly form a total set of landowners.