With the majority of patent applications receiving a restriction requirement from the USPTO (depending on the art unit), it is imperative that applicants not only understand how to navigate restrictions, but also how to evaluate their veracity.
One area of increasing importance is understanding how the Office restricts alleged species in an application. Some examiners consider any difference to qualify as defining a different species, but of course that is not the rule. Rather, the Office usually must establish that species are independent or distinct because the claims recite mutually exclusive characteristics of such species. See MPEP 806.04(f) Restriction Between Mutually Exclusive Species, which states:
Where two or more species are claimed, a requirement for restriction to a single species may be proper if the species are mutually exclusive. Claims to different species are mutually exclusive if one claim recites limitations disclosed for a first species but not a second, while a second claim recites limitations disclosed only for the second species and not the first. This may also be expressed by saying that to require restriction between claims limited to species, the claims must not overlap in scope
While this rule is somewhat applicant-friendly (in that it looks for "disclosure" of distinct limitations being only for certain species), the Applicant must be careful as the USPTO takes a broad reading of what it means to be mutually exclusive in other contexts, particularly in determining whether invention are "related". Specifically, MPEP 806.05 explains that "[r]elated inventions in the same statutory class are considered mutually exclusive, or not overlapping in scope, if a first invention would not infringe a second invention, and the second invention would not infringe the first invention." Here, the USPTO plays fast and loose with the meaning of "mutually exclusive" by switching to the broader term "not overlapping in scope." only then to switch yet again to a two-way distinctiveness test. This "related" invention test is often easier for the USPTO to establish than proving that there are mutually exclusive disclosed species in the application.
So, be careful when challenging an "invalid" election of species so as not to open the door to a "less invalid" restriction based on related inventions. Such a result actually occurred in a case petitioned by Applied Materials (14/462,303). While the applicant achieved a partial win in that case, the Office illustrated that it is more than willing to take any admission by the Applicant and use it to their advance in sustaining as much of a restriction as they can get away with.