Patent claims must be clear and definite so that everyone knows what they do and do not cover.  The statue, Section 112, requires that the claims particularly point out and distinctly claim the subject matter which the inventor regards as the invention. The Supreme Court made things quite "clear" as to to the requirement of claim scope clarity:

A patent is invalid for indefiniteness if its claims, read in light of the patent’s specification and prosecution history, fail to inform, with reasonable certainty, those skilled in the art about the scope of the invention.  Nautilus, Inc. v. Biosig Instruments, Inc. 

Despite this unequivocal directive, the USPTO recently decided that for Office, the requirement is different than what the Supreme Court directed. The PTAB, under the guise of the broadest reasonable interpretation (BRI) rule, applied a back-door analysis and skirted the Supreme Court's ruling. Specifically, in a precedential opinion (Ex Parte McAward, where Honeywell is the Assignee), the Board determined that for the USPTO, it was proper to require something different (and in practice more) than the statute and Supreme Court demands. According to the Board, the standard to be applied is whether a claim's meaning is "unclear." This profound conclusion was justified in the Board's view for numerous reasons, including that the Applicant is able to amend the claims during prosecution. Unfortunately, the Board's approach minimizes the context of the claim wording, particularly in light of the specification and the understanding of the person skilled in the art.

Time will tell whether this standard will be upheld by the Federal Circuit (who seems to be ever more willing to strike down overreach by the USPTO). Recently we have seen admissions by the USPTO leadership that they believe it is acceptable to stack the PTAB panels to obtain the decision that they prefer (which could have been done even in this case), and so hopefully there will eventually be consequences at the Federal Circuit for the USPTO's unfair actions.