If you do not already have a CSD file, you must create one. For detailed information about creating a CSD file, see the CICS® System Definition Guide.
You can create more than one CSD file, depending on your requirements. For example, you can have different CSD files for different systems, so that your test systems and production systems are separate from each other.
You can also share one CSD file between CICS releases; see Compatibility mode (CSD file sharing).
When the CSD file has been initialized, it contains a number of groups (all beginning with the letters 'DFH') containing related resource definitions, and one list, called DFHLIST. These definitions are supplied by CICS and are necessary for some system functions and for running CICS-supplied transactions.