Adaptive Server allows you to create user-defined datatypes, based on any system datatype, with the sp_addtype system procedure. You cannot create a user-defined datatype based on another user-defined datatype, such as timestamp or the tid datatype in the pubs2 database.
The sysname datatype is an exception to this rule. Though sysname is a user-defined datatype, you can use it to build user-defined datatypes.
User-defined datatypes are database objects. Their names are case-sensitive and must conform to the rules for identifiers.
You can bind rules to user-defined datatypes with sp_bindrule and bind defaults with sp_bindefault.
By default, objects built on a user-defined datatype inherit the user-defined datatype’s null type or IDENTITY property. You can override the null type or IDENTITY property in a column definition.