User defined data types are based on system data types. They should be used when multiple tables need to store the same type of data in a column and you need to ensure that all these columns are exactly the same including length, and nullability.

Parameters for user defined datatype:

Name

System data type on which user defined data type is based upon.

Nullability

For example, a user-defined data type called post_code could be created based on char system data type.