The dummy dataset is coded on the DD ststement to test the execution of the pgm without actually reading or writing into the dataset.It basically
simulates the presence of the file even without it actually existing.
//ddname DD DUMMY
if the DUMMY FILE IS READ the O.S sends an EOF indicator to the pgm being executed.This will result the pgm to execute based on it's logic without actually reading any data.
If write is requested the file being imaginary is simply ignored.