What are user defined datatypes and when you should go for them?

User defined datatypes lets us extend the base SQL Server datatypes by providing a descriptive name, and format to the database.

Take for example, in your database, there is a column called Flight_Num which appears in many tables. In all these tables it should be varchar(8).

In this case you could create a user defined datatype called Flight_num_type of varchar(8) and use it across all your tables.

refer to sp_addtype, sp_droptype.