Additional rules for modifying DD parameters

The following additional rules apply for modifying DD parameters:

To nullify all parameters but the DCB parameter, code DUMMY on the overriding DD statement.
Special rules apply when overriding a DCB parameter:
- Code only the keyword subparameters to be changed; the other DCB subparameters remain unchanged.
- If a positional subparameter is needed, code it, regardless of whether it appears in the overridden DCB parameter. If a positional subparameter is not needed or is to be nullified, omit it from the overriding DCB parameter.
- To nullify the entire DCB parameter, nullify each subparameter appearing in the overridden DCB parameter.
To nullify a DUMMY parameter on the procedure statement, code one of the following on the overriding statement:
- A DSNAME parameter with a name other than NULLFILE
- A SYSOUT parameter
- A * or DATA parameter
- A SUBSYS parameter.