Claim construction serves as the foundation for determining both the patentability of a patent application and the scope of protection. Rule 19, Paragraph 4 of the Enforcement Rules of the Patent Act stipulates that a technical feature in a claim for an invention which consists of a combination of multiple technical features may be expressed in means-plus-function or step-plus-function language, and the scope of such claim shall be interpreted to cover the corresponding structures, materials or acts described in the specification and equivalents thereof. By focusing on the functional concepts of elements in a combination instead of exhaustively detailing their possible structural variations, practitioners can use means-plus-function or step-plus-function language as an efficient and flexible way to protect the core functionality of inventions without being tied to specific implementations, ensuring that the intellectual property rights remain relevant as technology evolves. This article explores the general criteria and judicial practice concerning the interpretation of step-plus-function or means-plus-function claims (hereafter “MPF claims”).

II.      General Criteria

To determine whether a claim should be construed as an MPF claim, the Patent Examination Guidelines[1] indicate the three-prong test to be satisfied:
 

(a)  Linguistic syntax: The phrase “means (or device) for …” or “step for …” is used in the claim to recite a technical feature.

(b)  Specific functional recitation: The phrase “means (or device) for …” or “step for …” must be paired with a clearly recited and specific function.

(c)  Absence of structure: The recitation with the phrase “means (or device) for …” or “step for …” shall not define any complete structure, material or action sufficient to achieve the function.
 

In addition, since the scope of an MPF claim may cover the corresponding structures, materials or acts described in the specification and equivalents thereof, the disclosure in the specification is critical for the interpretation of an MPF claim. In particular, the Patent Examination Guidelines state that if the specification does not recite a structure, material or action corresponding to the function and its scope of equivalence, or if the recited structure, material or action in the specification is overly generic, such that a person having ordinary skill in the art (PHOSITA) is unable to determine a structure, material or action corresponding to the function from the specification, the claim may be rendered unclear. Hence, the disclosure of the specification affects not only the supportability but also the clarity of MPF claims.

 

Furthermore, in patent infringement assessment, according to the Directions for Determining Patent Infringement[2] published by the Taiwan Intellectual Property Office (TIPO), a technical feature in a claim expressed with MPF language is analyzed through the following steps:
 

The exact function recited in the claim must be identified. The literal scope of an MPF element is strictly limited to the function recited in the claim. If an accused article performs a function that is merely “substantially similar” but not identical, it cannot satisfy the literal requirements of the MPF element, and the patentee must instead rely on the Doctrine of Equivalents.

The identification of corresponding structures, materials or acts that perform the function recited in an MPF claim is based on the specification. The specification should disclose a clear link or explicit relationship between the structures, materials or acts and the function. Furthermore, the corresponding structures, materials or acts are restricted to the minimal components absolutely necessary to perform the recited function, excluding optional, assembly-related, or auxiliary structures that do not directly perform the recited function.

After identifying the structures, materials or acts described in the specification that correspond to the function recited in the claim, if the corresponding structures, materials or acts of the accused article differ from them but fall within the scope of equivalents, the two technical features shall still be deemed identical.
 

 

“Equivalent scope” means that, relative to the structures, materials or acts described in the specification corresponding to the function recited in the claim, the accused infringing object has no substantial difference at the time of patent filing, or that it performs the “same” function in a substantially identical way to achieve a substantially identical result. Moreover, the phrase “no substantial difference at the time of patent filing” means that the corresponding structures, materials or acts are interchangeable at the time of patent filing. That is, such interchangeability was known to a PHOSITA at that time, and the function produced after the substitution remains the “same.”

III.    Court Practice

The following judgment provides insight into the Taiwan Intellectual Property and Commercial Court’s (IPCC’s) approach to interpreting MPF claims.

 

IPCC 2025 Xing-Zhuan-Su-Zi No. 16 Judgment[3] (issued in October 2025)
Background
The case involves patent TWI629025 (entitled “Slide Rail Assembly”) co-owned by King Slide Works Co., Ltd. and King Slide Technology Co., Ltd. (the patentees). The independent claims of this patent are directed to slide rail assemblies, comprising: a first rail; a second rail movable relative to the first rail; a third rail movable relative to the second rail; a locking mechanism (or locking member) configured to be in a first state or a second state, wherein the locking mechanism in the first state is configured to prevent the movement of the second rail relative to the first rail; and an operating member configured to be manually operated to switch the locking mechanism from the first state to the second state.

Upon an invalidation action requested by a third party, the TIPO made an invalidation decision revoking the patent on the grounds that the claimed inventions did not possess novelty and/or an inventive step over the prior art documents (including TWM250596 as Exhibit 8). The patentees were dissatisfied with the TIPO’s invalidation decision and filed an administrative appeal with the Petitions and Appeals Committee of the Ministry of Economic Affairs (MOEA). The Petitions and Appeals Committee sustained the TIPO’s invalidation decision, and thus the patentees (plaintiffs) filed an administrative lawsuit seeking revocation with the IPCC.

Opinions of the Court
In the administrative litigation, one of the disputes was whether the claims of TWI629025 involve MPF language. The patentees argued that the terms “locking mechanism,” “locking member” and “operating member” in Claims 1, 3, 8 and 12 constitute means-plus-function limitations. Through this argument, the patentees sought to rely on corresponding structures disclosed in the specification to narrow or redefine the scope of the claims, thereby bypassing the risk of invalidation.

Nonetheless, the IPCC did not agree with the patentees’ argument and opined that the terms “locking mechanism,” “locking member” and “operating member” are well-known to a person having ordinary skill in the sliding-rail field and convey both functional and implicit structural meanings. Further, the terms “locking,” “blocking” and “operating” are also described in the prior art section in the specification of TWI629025, indicating that the structural meanings of these terms were known to a PHOSITA. Because the terms “locking mechanism,” “locking member” and “operating member” in the claims imply structures sufficient to achieve the functions, the claims do not meet the three-prong test for determining MPF claims.

The IPCC further identified the following fundamental inconsistency in the patentees’ argument: Claim 1 recites a locking mechanism; and Claim 2, which depends on Claim 1, specifies that the locking mechanism contains a locking element movably mounted on the second rail, and an elastic element providing an elastic force to the locking element. If the “locking mechanism” in Claim 1 were interpreted as an MPF term, it would be restricted to the specific embodiment in the specification. This interpretation would make the scope of Claim 1 identical to that of Claim 2, violating the principle of claim differentiation and the conciseness requirement.

Consequently, the IPCC concluded that the claimed inventions of TWI629025 lack novelty and an inventive step and dismissed the patentees’ request for revocation of the invalidation decision.

However, the aforementioned IPCC judgment is not final and binding. The case is currently pending on appeal before the Supreme Administrative Court.

IV.    Conclusion

Based on the general criteria for the interpretation of MPF claims, if patent applicants intend to define features in claims by MPF limitations, in addition to carefully drafting claims to meet all the conditions of the three-prong test, they should also devote substantial effort to drafting the specification, disclosing as many corresponding structures, materials or acts as possible. Close strategic coordination between claim drafting and specification disclosure is therefore essential.

Additionally, the aforementioned IPCC judgment indicates that if a functional term implies an established structure in the relevant technical field, the functional term may not constitute an MPF limitation. Considering that the interpretation of MPF claims may diverge during litigation proceedings, instead of simply employing MPF claims, it is advisable to strategically combine MPF limitations with structural limitations to establish more comprehensive patent protection.

References