Rules for modifying OUTPUT JCL or DD parameters

The override operation merges the parameters from an overriding statement with those in the overridden statement. Follow these rules in coding overriding statements:

You can code more than one change on an overriding statement.
You can code modifying parameters in any order on an overriding statement.
Code an entire overriding parameter even when changing only part of that parameter.
If you code a parameter on an overriding statement that is not on the procedure statement, the override operation adds it to the procedure statement.
Nullify a parameter by not coding a value after the equal sign. Omitting the value causes the system to treat the keyword as if it had been removed from the procedure statement. This is the only way to nullify keywords that do not permit a null parameter value.

If you add a parameter that is mutually exclusive with a parameter on a procedure statement, the override operation automatically nullifies the procedure parameter. This is the only exception to the rule that the only way to override a parameter is to specify it explicitly.

Example:  If a DD statement within a procedure reads:

             //ddname DD DSN=FRED,DISP=SHARE,UNIT=TAPE,VOL=SER=111111

          and you wish to modify that DD statement to read in
          data set GEORGE, which is cataloged to a DASD volume,
          it is NOT sufficient to specify:

             //stepname.ddname DD DSN=GEORGE

          Instead you must specify:

             //stepname.ddname DD DSN=GEORGE,UNIT=,VOL=

          This nullifies the UNIT and VOLUME information, allowing
          the system to retrieve that information from the catalog.
          (An overriding DD statement without those parameters would
          cause the system to find data set GEORGE on tape volume
          serial 111111.)