In the Adding a new book use case, several phrases appear to be important concepts that should be documented. For example, Dewey number is domain-specific terminology that should be defined for the benefit of non-librarians. A description can also help us understand how we need to represent it in the system.
The Dewey number is an attribute of a catalog entry, so it's probably not a thing in its own right; therefore, we'll use a Concept element to define it. To do this, follow these steps:
A Dewey number is a numerical classification identifying a book's subject matter.
Another phrase that looks like a good candidate for a Concept element is serial number. Like the Dewey number, the serial number is an attribute of a catalog entry, so it is probably not a thing; however, it's an important concept that probably should be recorded. But suppose that, in trying to define serial number, we find that our use case does not reflect correct terminology?