Objections to the clarity of claims (here equivocality of the term "average particle size") or any consequent requests for amendment are only relevant to opposition proceedings in so far as they can influence the decisions on issues under Art. 100 EPC or arise in relation to the subject-matter to be amended as a consequence of such issues. This was the view taken by the board in T 127/85 (OJ 1989, 271), to which it added that it would be an abuse of opposition proceedings if the patent proprietor were allowed merely to tidy up and improve the disclosure by amendments not specifically necessitated by the grounds advanced for the opposition, even if those amendments were to comply with Art. 123 EPC. See also T 89/89 and T 324/89.
According to the established case law of the boards of appeal, Art. 84 EPC is an EPC requirement that has to be taken into account in opposition proceedings whenever the patent proprietor files any amendments (within the limits set in G 3/14, OJ 2015, A102; see in this chapter IV.C.5.2.2). However, Art. 84 EPC is not itself a ground for opposition under Art. 100 EPC. Irrespective of how the granted claims are formulated, and be they dependent or independent, an opposition cannot be lodged on the basis that they lack clarity (T 23/86, OJ 1987, 316; T 565/89; T 89/89; T 16/87, OJ 1992, 212; T 1835/08; T 1855/07). The board in T 89/89 followed the findings in decision T 295/87 (OJ 1990, 470) according to which amendments during opposition proceedings should only be considered as appropriate and necessary in the sense of R. 57(1) and 58(2) EPC 1973 (R. 79(1) and R. 81(3) EPC) and therefore admissible if they can fairly be said to arise out of the grounds of opposition. It therefore regarded the proposed amendments (consisting in correcting the two-part form of claim 1 and clarifying the meaning of its last feature but not relevant for the decision on the grounds for opposition) as inadmissible. In T 792/95, the board confirmed the principle that, in opposition proceedings and subsequent appeal proceedings, only such amendments could be made as were necessary in the light of the grounds for opposition under Art. 100 EPC (citing T 23/86, OJ 1987, 316; T 127/85, OJ 1989, 271 and T 168/85).
In T 113/86 the board of appeal followed this principle, ruling against the admissibility of amendments proposed by the patent proprietor which were not necessary to take account of the grounds for opposition, if there were the slightest possibility of a different interpretation being given to the patent specification before and after the amendments. The protection conferred by the patent would actually be extended if, as a result of amendments to clarify the granted claims, the claims could be more widely construed than a court would have construed them by the application of Art. 69 EPC. However, the board was of the opinion that the removal of an inconsistency between a claim and the description should be allowed if the inconsistency arose from an error, provided that the error was so obvious to a skilled person in the light of the patent specification as a whole that an interested third party could have anticipated the extent of protection conferred by the amended claim. In these circumstances the request for the correction of an error did not represent an abuse of opposition proceedings. Moreover, the removal of the discrepancy was in the interests of legal certainty.
Where a deficiency results from a discrepancy between the description and the claim, lack of clarity in the invention's disclosure can be objected to because that requirement comes under Art. 83 EPC, not Art. 84 EPC, and may therefore be taken into consideration in opposition proceedings (see e.g. T 175/86).