Claims of a patent are what define the invention that is protected. One way to define the invention is in terms of its pure function, without regard to the specific structure used to perform that function.  This approach is authorized by 35 U.S.C. Section 112, sixth paragraph (pre-AIA) and 35 U.S.C. Section 112(f) (post-AIA).  However, according to U.S. case law, means claims can be interpreted narrowly depending on the structure disclosed in the specification.  In contrast, foreign jurisdictions generally interpret means claims to provide broad coverage.

An interesting aspect of the statute authorizing means claims is that it simply authorizes the use of a certain claim type (e.g., means claims) and defines how claims that fall under the section are interpreted. As such, it cannot form the basis of a rejection by an Examiner. A recent example of this is illustrated in the appeal of US 13/168,034.

The case relates to a fuel cell control system and recited a claim element setting forth certain features of the controller in means format:

The Examiner had rejected the means claims under 35 U.S.C. Section 112, sixth paragraph, as being in an improper format for providing too much structure and not just providing function. The PTAB reversed, noting that:

Further, the PTAB also confirmed that the means claim was properly drafted and did not recite a structure that performs the function.

So, if you are facing rejections under 35 U.S.C. Section 112, sixth paragraph, there is likely a strong argument that the rejection is per se improper.