When a symbol is a positional parameter followed by other parameters in the statement, follow the symbol with a period instead of a comma. For example:

   //DS1   DD  &POSPARM.DSNAME=ATLAS,DISP=OLD

If &POSPARM is nullified, the statement appears as:

   //DS1   DD  DSNAME=ATLAS,DISP=OLD

When assigning a substitution text to &POSPARM, include the comma:

   POSPARM='DUMMY,'