There is an interesting paradox between the actual wording and the general interpretation of 35 USC §121, which affects prosecution history estoppel during patent prosecution.  35 USC §121 states, in part, “If two or more independent and distinct inventions are claimed in one application, the Director may require the application to be restricted to one of the inventions.”  Parsing this sentence for meaning, one might conclude that a restriction requirement can be issued by the USPTO if a patent application claims two inventions that are both independent and distinct.  In other words, for a patent application that claims two inventions that are distinct but not independent, i.e., are different but related in some manner, 35 USC §121 would not authorize issuance of a restriction requirement.  But, that is not how the statute is interpreted.

MPEP (Manual of Patent Examining Procedure) 806 states, in part:
(A) Where inventions are independent (i.e., no disclosed relation therebetween), restriction to one thereof is ordinarily proper, MPEP § **>806.06<.>
(B) Where inventions are related as disclosed but are distinct as claimed, restriction may be proper.
(C) Where inventions are related as disclosed but are not distinct as claimed, restriction is never proper.
(D) A reasonable number of species may be claimed when there is an allowable claim generic thereto. 37 CFR 1.141, MPEP § 806.04.

Thus, according to the MPEP, a patent application that claims two (or more) inventions that are either distinct or independent can properly be restricted.  This interpretation is further confirmed in MPEP 806.04(b) Species May Be Independent or Related Inventions.  So, attempting to argue that a restriction requirement is improper under 35 USC §121, i.e., traversing the restriction requirement, by stating that two groups of claims are directed to embodiments that are not both distinct and independent would be a big mistake.

In my experience, the USPTO seems to be increasing the issuance of restrictions during patent prosecution.  It is quite common for an Office Action to require restriction when claims are directed to a method and a system, or a method and an apparatus, but we are also seeing restriction requirements when claims are directed to various systems or various apparatuses.  For reasons discussed in our October 8, 2013 posting, “Restriction Requirement: to Traverse or Not to Traverse?”, it is often preferable to select a group of claims without traversing the restriction requirement, and thus avoid placing statements in the prosecution history that might later become damaging.  This strategy is often acceptable to clients who want to build a patent portfolio.  A parent patent application, facing a restriction requirement, can spawn one or more divisional applications, and possibly also one or more continuation applications.  Traversing the restriction requirement, even with statements that might appear innocent at the time, could place these later applications at risk.