c) Full and concise reasoning 

In T 222/85 (OJ 1988, 128) the board held that the third requirement was only satisfied if the contents of the notice of opposition were sufficient for the opponent's case to be properly understood on an objective basis, from the point of view of a reasonably skilled person in the art to which the opposed patent relates. The board reasoned that the purpose of the third requirement of R. 55(c) EPC 1973 (R. 76(2)(c) EPC), in combination with the first two requirements, was to ensure that the notice of opposition set out the opponent's case sufficiently so that both the patentee and the opposition division knew what that case was. Whereas the requirements of R. 55(a) and (b) EPC 1973 (R. 76(2)(a) and (b) EPC) and the requirements (1) and (2) of R. 55(c) EPC 1973 could be considered as formal in nature, requirement (3) of R. 55(c) EPC 1973, in combination with Art. 99(1) EPC, was substantive in nature, and called for reasoning which went to the merits of the opponent's case. A well-drafted opposition ought to contain reasoning that was full but concise. And in general, the less reasoning that a notice of opposition contained, the greater the risk that it would be rejected as inadmissible (similarly T 925/91, OJ 1995, 469; see also T 2/89, OJ 1991, 51; T 448/89, OJ 1992, 361; T 545/91, T 204/91). According to the board, the question whether a particular notice of opposition meets the minimum substantive requirements of Art. 99(1) EPC 1973 and R. 55(c) EPC 1973 could only be decided in the context of the particular case (since various relevant factors, such as the complexity of the issues raised, vary from case to case) – see also e.g. T 534/98, T 1097/98, T 934/99, T 426/08.

In T 134/88 the board found that allegations which could not be subsumed under one of the opposition grounds had to be left out of consideration (see also e.g. T 521/00).

In T 623/18 the board qualified these principles established in T 222/85 and T 134/88 by emphasising inter alia that there was no basis for making the admissibility of the opposition dependent on substantive questions, such as whether an objection relates merely to a lack of clarity (see the abstract in this chapter IV.C.2.2.8 g)).

In T 204/91 the board stated in particular that the term "indication" in R. 55(c) EPC 1973 (R. 76(2)(c) EPC) had to be construed as requiring more than a mere hint at a number of possible attacks upon the patent and at the likely support for each such possible attack, which indication or hint might then be augmented by the subsequent late filing, possibly even at the appeal stage, of further evidence, arguments or other matter – even of fresh grounds of objection. Rather, the scope and depth of "indication" needed to be such as to enable the patentee and the opposition division to see clearly just what attack was being mounted against the patent, and what evidential support was being adduced for that attack. In other words, the patentee and the opposition division had to be put in a position of understanding clearly the nature of the objection submitted as well as the evidence and arguments in its support. This required the elaboration of the relevant circumstances of the case to such an extent that the patentee and the opposition division were able to form a definitive opinion on at least one ground for opposition raised, without the need to make further investigations (see T 453/87 and T 279/88; see further e.g. T 1069/96 and T 426/08).

The fact that a patentee must be able to understand, without undue burden, the case made against his patent in the notice of opposition does not, however, exclude the possibility that he may have to undertake a certain amount of interpretation (T 199/92; see also T 1553/07, T 265/16; see however T 1082/00, which highlights the limits of this principle and refers to T 204/91).