Three requirements, then two more
Section 112 is often spoken of as one thing. It is at least five, and a rejection is easier to answer once it is clear which one is at issue.
Under § 112(a), a specification must satisfy three separate requirements: written description, enablement, and best mode. The MPEP is explicit that these are distinct, and conflating them is a common source of confusion in both rejections and responses.
Under § 112(b), claims must particularly point out and distinctly claim the subject matter regarded as the invention — the definiteness requirement.
Under § 112(f), a limitation drafted in means-plus-function form is construed to cover the corresponding structure described in the specification and its equivalents. That is a claim-construction rule rather than a separate patentability requirement, but it frequently drives both indefiniteness and written-description outcomes.
Written description versus enablement
These are the two most frequently confused, and the distinction is worth stating precisely.
Written description asks whether the specification conveys to a person of ordinary skill that the inventor had possession of the claimed subject matter at the time of filing. Ariad Pharm., Inc. v. Eli Lilly and Co. confirmed that this is a requirement separate from enablement. It bites hardest on claims broader than what was actually described — a genus claimed on the strength of a few species, or a functional limitation with no disclosed structure or mechanism achieving that function.
Enablement asks whether the specification teaches a person of ordinary skill how to make and use the full scope of the claim without undue experimentation. The In re Wands factors are the standard framework for whether experimentation is undue: the quantity of experimentation, the amount of direction given, the presence of working examples, the nature of the invention, the state of the prior art, the relative skill in the art, the predictability of the art, and the breadth of the claims.
The practical difference: a specification can enable something it never described as the invention, and can describe something it does not enable. An examiner citing "lack of support" without identifying which requirement is being applied is worth asking to clarify, because the two responses differ entirely.
Definiteness
Nautilus, Inc. v. Biosig Instruments, Inc. set the standard applied by courts: a claim is indefinite if, read in light of the specification and the prosecution history, it fails to inform those skilled in the art about the scope of the invention with reasonable certainty.
Examination applies a lower threshold than a court does, because claims can still be amended during prosecution and the Office resolves ambiguity by requiring clarification rather than by construing around it. § 2173 collects the recurring categories: terms of degree with no standard for measuring them, antecedent basis problems, claims whose scope is unclear relative to the disclosure, and functional language that does not delimit what is covered.
When § 112(f) is triggered
Use of the word "means" creates a rebuttable presumption that § 112(f) applies; its absence creates the opposite presumption. Williamson v. Citrix Online, LLC materially weakened the second presumption, holding that a limitation reciting a generic placeholder for structure together with a function can invoke § 112(f) even without the word "means."
This matters beyond construction. Once § 112(f) applies, the specification must disclose corresponding structure for the recited function, and a failure to do so produces an indefiniteness problem — one that cannot be cured by amendment if the structure was never described. For computer-implemented functions, the disclosed structure is generally the algorithm the specification provides, not the general-purpose processor executing it.
Reading order
Start at § 2161, which lays out that the § 112(a) requirements are separate, then go to the section for the requirement actually at issue: § 2163 for written description, § 2164 for enablement, § 2165 for best mode. § 2173 covers definiteness. § 2181 covers identifying and interpreting a § 112(f) limitation — read it first if the rejection turns on whether § 112(f) was invoked at all.