You can use domain definitions to define and name domains for table columns.
Each domain definition has a name that is unique within the overall database system. If a domain has been defined without an owner being entered, the database system searches for the domain in the catalog parts in the following order:
1. Catalog part of the current owner
2. Catalog part of the database administrator who created the current user
3. Catalog part of the database system administrator
SQL Reference Manual, Domain Name
SQL Tutorial, Domains