According to the case law of the boards of appeal a criterion to be taken into account is whether a late-filed document is prima facie highly relevant and whether there is proper justification for its late filing to forestall tactical abuse. Such material should be prima facie highly relevant in the sense that it can reasonably be expected to change the eventual result and is thus highly likely to prejudice the maintenance of the European patent (see T 1002/92, OJ 1995, 605; T 212/91; T 931/06; T 501/09; T 1306/09 and T 2542/10). See also under chapter IV.C.4.5.3 "Prima facie relevance".

In T 1002/92, the board stated that the criteria governing the admission of late-filed facts, evidence and arguments are more restrictive and stringent in proceedings before the boards of appeal than in opposition proceedings at first instance. In contrast to the first-instance proceedings, the appeal procedure is a judicial procedure and therefore "less investigative". Therefore, as regards proceedings before the boards of appeal, new facts, evidence and related arguments which go beyond the "indication of the facts, evidence and arguments" presented in the notice of opposition pursuant to R. 55(c) EPC 1973 in support of the grounds of opposition on which the opposition is based should only very exceptionally be admitted into the proceedings in the appropriate exercise of the board's discretion and if such new material was prima facie highly relevant.

In R 6/17 the Enlarged Board noted that the prima facie relevance of a document is not listed explicitly in Art. 13(3) RPBA 2007 dealing with amendments of a party’s case after the summons to oral proceedings. However, the list of criteria in Art. 13(1) RPBA 2007 is non-exclusive, and the criterion of prima facie relevance has been applied by some boards of appeal as one criterion amongst others. The prima facie relevance is certainly neither the most relevant nor does it stand alone, i.e. without interdependence on other criteria. The interest in procedural expediency and procedural economy is also expressed in the common approach not to admit late documents which, prima facie, are no more relevant than what is already on file.

In T 2054/11 the board observed that, once oral proceedings on appeal had been arranged, and especially just before or during those proceedings, whether or not late-filed amendments to a party's case could be considered no longer depended in practice on whether they were relevant (see Art. 13(3) RPBA 2007).

In T 2576/12 the board pointed out that, when exercising its discretion under Art. 12(4) RPBA 2007, it could make admitting prior art filed for the first time with the grounds for appeal dependent on whether it was prima facie (highly) relevant but that it did not have to, because otherwise there would be nothing to stop an opponent from always withholding (highly) relevant prior art until filing its grounds for appeal in the expectation that it would be admitted because of its relevance (see also T 724/08).

In T 887/11 the board admitted the (new) evidence as well as the offered witness testimony submitted with the appeal grounds into the proceedings because of its high relevance. In T 182/09 likewise the board admitted a highly relevant document into the proceedings (see also T 1404/10, T 605/11).

In T 340/12 the board admitted the late-filed document into the proceedings and stated that in this case the prima facie high relevance of the document should take precedence over the procedural aspect of its late filing.

In T 158/14 the board was satisfied that the opponent provided a reasonable justification why D9 and D10 were not filed during the nine-month opposition period: these documents were Japanese utility models having no English abstract and were found by the opponent only by chance after discussing the appealed decision with one of its Japanese clients. The board saw no indication that the opponent deliberately withheld D9 and D10 for tactical reasons and D9 and D10 were highly relevant with regard to the patentability of the claimed subject-matter.

In T 931/06 the board stated that when assessing inventive step, a late filed document may be considered if it belongs to the same or a closely related technical field and discloses subject-matter conceived for the same or a similar purpose as the contested patent. In the light of T 1002/92 such a document would then be prima facie relevant in the sense that it might prejudice the maintenance of the patent, at least in unamended form.

In T 1105/04 the board referred to T 736/99 and admitted a late-filed document into the proceedings, because it was considered by both parties as the closest prior art document.

In T 609/99 the board stated that despite the proprietor's objection to the admission of evidence which had been disregarded in the opposition procedure pursuant to Art. 114(2) EPC 1973, the board might, in certain circumstances, take account of this evidence without any implication that it was prima facie highly relevant. This exercise of discretion was not inconsistent with the criteria set out in T 1002/92 (OJ 1995, 605). In the circumstances of the case at issue the board's admission of evidence had not extended the legal and factual framework.

In T 2335/12 the board concluded that the experimental evidence disclosed in document D32 had been filed late by the appellant for unjustified reasons and lacked prima facie relevance.