A patent generally covers a single invention. Often, however, a patent application will be filed that arguably covers multiple inventions. The Patent Office uses Restriction Requirements as discretionary tools to limit the examination of a patent to only one invention. In order to issue a restriction requirement, the Patent Examiner must find that two independent or distinct inventions have been claimed and that there would be a serious burden on the Examiner to examine the application as filed.
In the chemical practice (TC1600), the patent office has recently taken the initiative to to improve the quality and consistency of restriction practices. The PTO’s 5-step action plan is available online. Recently, the PTO made the Examiner training materials available on the Web in PDF format. (Thanks to Steve Hird for the links).
The training materials note that the strongest reasons for restriction involve either (i) separate technology classification; (ii) separate status in the art; or (iii) divergent field of search.
Tip: Reviewing the training materials may be a useful way to avoid or overcome restrictions. For example, by arguing that claims are "linked," an applicant may be able to persuade the Examiner to remove a Restriction requirement.
Steve Nipper also pointed me to another great resource for responding to restriction requirements: “Amendment and Response in the Mechanical Arts [pdf]” by Brian I. Marcus.