Letting the system assign the specific tracks is most frequently used. You specify only how space is to be measured — in tracks, cylinders, blocks, or records — and how many of those tracks, cylinders, blocks, or records are required.

The SPACE parameter has no meaning for tape volumes; however, if you assign a data set to a device class that contains both direct access devices and tape devices, for example, UNIT=SYSSQ, you should code the SPACE parameter.

If you code the SPACE parameter on a DD statement that defines an existing data set, the SPACE value you specify temporarily overrides the SPACE value used to create the data set. For example, a data set created with SPACE=(CYL,(5,1)) causes 5 cylinders to be allocated to the data set, and, if it needs more space, it can obtain 1 additional cylinder.

Suppose, though, that there is one particular job that specifies DISP=MOD and will write many records to this data set. JCL for this job can define, for example, SPACE=(CYL,(5,10)) to obtain an additional 10 cylinders instead of just 1 cylinder. The override, however, is in effect only for the single step of the job which specifies the overriding SPACE parameter. Any other job or step within the same job that requires a secondary extent and does not have a SPACE parameter override gets just the 1 additional cylinder specified in the JCL that created the dataset.