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Domains, also sometimes called "User Defined Datatypes" are a powerful construct to create more uniform table definitions and can include check constraints.
The Domain Editor offers you the possibility to create, alter and drop domains. If you add a domain, any opened Table Editor will add these domains to it's domain drop down box. So you can quickly create a new domain while creating a table.
Below is an example, take from the Firebird Domain Editor:
Firebird Domain Editor
The "{..}" button will turn the current line or text selection in the Check Constraint box into a comment, or remove the comment from the current line.