The IEFDDSRV macro enables authorized callers to find the devices associated with an allocation request. For example, for a JCL input statement that requests the allocation of one or more data sets, you can use IEFDDSRV to obtain the addresses of the UCBs for the devices containing those data sets. When you specify that an above 16 megabyte UCB not be captured during dynamic allocation, you must use the IEFDDSRV macro to retrieve the UCB address.
You can find more specific information about the IEFDDSRV macro in z/OS MVS Programming: Assembler Services Reference IAR-XCT