Domains in the Logical Data Model
You should always define and assign domains to your table attributes during the logical design phase of your database design.
Domains can be bounded specifically, nonspecifically, or both simultaneously. For example, you can define the employee number domain to be both not null (a nonspecific boundary) and constrained within a restricted range such as greater than 100 and less than 1 000 000. Unfortunately, you cannot specify CHECK or any other types of constraints on UDT columns, which lessens their usefulness for defining domains.