What is the difference between an attribute and a domain?

The attributes are the properties of an entity that characterize and describe it. Figure below shows the BOOK entity b1 described by the attributes Book_title, Price (in dollars), ISBN, Year, Page_count and Category. Each attribute can accept a value from a set of permitted values, which is called the domain or the value set of the attribute. For example, the domain of the attribute Page_count is a set of positive integers. On the other hand, the category of a book can be a textbook, a language book or a novel. Thus, the domain of the attribute Category is {Textbook, Language book, Novel}. Only one of these three permitted values can be assigned to the category attribute.