Comments are enclosed between pre-processor lines %comment and %end. These lines are then ignored when the file is being pre-processed, either for running the job or for the manual-page. You cannot, however, comment-out a part of the manual page.
Since comments are processed before a job is created they can be placed anywhere in the script. Just remember that comments do not nest and that you cannot use comments inside manual pages. (You can, of course, change the %manual to be %comment)
The following extract is an example of using comments:
mars << EOF RETRIEVE, PARAM=10U/10V,DATE=..., %comment temp mod by OP / 1.1.2012 BC TARGET="/xx/yy/zz", %end TARGET="zz", END EOF And the following is the corresponding output when pre-processed. mars << EOF RETRIEVE, PARAM=10U/10V,DATE=..., TARGET="zz", END EOF