A user-defined data types in SQL are Domain Type. You can use the domain instead of a built-in SQL data type. To know the difference between user-defined and built-in data types check our post. You can use the domain instead of a built-in SQL data type when declaring a column. That column will then be restricted only to use the same domain. Some good definitions of Domain type in SQL are found HERE.
To read about different types of Domain in SQL Visit https://hecodesit.com/domain-types-in-sql-structured-query-language/