As explained here by George,
In the U.S. a restriction requirement can be issued by an examiner who feels a claim set covers more than one invention. One criteria they can use is to assert “ undue search burden” and identify claims as falling into two different class/subclass groupings.
Now, if USPTO rejects a submission for that clause what are the available legal actions the applicant can take? If applicant splits and re-submit multiple patents would that cost them extra money?