If two or more independent and distinct inventions are claimed in a single application, the examiner may require the applicant to elect (designate) a single invention to which the claims will be restricted (limited to). This requirement is known as a requirement for restriction (also known as a requirement for division). Such requirement will normally be made before any action on the merits; however, it may be made at any time before final action (final rejection).

Source: U.S. Patent and Trademark Office.