One of the most important concepts associated with UDTs is strong typing. Strong typing guarantees that only functions and operators defined on the UDT can be applied to its instances.
Strong typing is important to ensure that the instances of your UDTs are correct. For example, if you have defined a function to convert US dollars to Canadian dollars according to the current exchange rate, you do not want this same function to be used to convert euros to Canadian dollars because it will certainly return the wrong amount.
As a consequence of strong typing, DB2 does not allow you to write queries that compare, for example, UDT instances with instances of the UDT source type. For the same reason, DB2 will not let you apply functions defined on other types to UDTs. If you want to compare instances of UDTs with instances of another type, you have to cast the instances of one or the other type. In the same sense, you have to cast the UDT instance to the type of the parameter of a function that is not defined on a UDT if you want to apply this function to a UDT instance.