hi Chandu,
the length field is optional, in case of custom jobs like redwood script job or cmd in your case.
Length refers to the restriction on character length of the value of the parameter.
For example, in case of an ABAP job, the parameter VARIANT has length value set to 14, meaning that the ABAP variant name cannot be longer than 14. That is a restriction in SAP itself.
So, every parameter you define in your custom CMD job definition, you can restrict (if you want) the length of the values provided. For example, the maximum character length of windows directory path can be 260, you can put that restriction so that someone doesn't (accidentally) cross the limit, because then the job is bound to fail. When you add the restriction, the job will not submit throwing the error that the condition for length field is not satisfied.
Basically you are adding a pre-validation that all supplies parameter values are correct.
thanks
Nanda