What does it mean if a claim is "impervious" to a double-patenting rejection?

When a claim is described as "impervious" to a double-patenting rejection, it indicates that the claim is distinct enough that it cannot be subject to a double-patenting rejection in relation to a previously issued patent. Specifically, this indicates that the claim can only be effectively pursued within the context of a divisional application. A divisional application is required when the original application contains multiple inventions, and one of those inventions is pursued in a separate application to avoid issues of double patenting. This means the claim in question does not overlap sufficiently with prior claims of a parent application, allowing it to maintain its own patentability.

In contrast, other potential interpretations do not accurately capture the nuances of patent rejections. The notion of being immune to additional review does not apply here, as all claims are subject to examination, albeit under different grounds. Moreover, claims that cannot stand on their own or have been previously accepted without conditions imply other constraints or scenarios that are not relevant in the context of double-patenting rejections. Hence, understanding that a claim being "impervious" means it is uniquely positioned to be pursued in a divisional application clarifies its distinctiveness in the patent examination process.