FBEs and IRDs are invoked by CICS® VT during the initial data migration process. In general, an FBE does not need to consider the type of call being processed. It either builds DB2® data from VSAM data, or the other way around.
In virtually every case, an IRD must include functionality specifically for initial data migration as well as for PUT/WRITE/REWRITE calls. An IRD is sensitive to the type of call being processed.