What means "UDT" ?

Beginning with SQL Server 2005, you can use user-defined types (UDTs) to extend the scalar type system of the server, enabling storage of CLR objects in a SQL Server database. 

Because UDTs are accessed by the system as a whole, their use for complex data types may negatively impact performance. 

CLR UDT at msdn : http://msdn.microsoft.com/en-us/library/ms131120.aspx