Art. 112a(3) EPC makes it clear that the petition for review is an extraordinary legal remedy the filing of which does not affect the force of res judicata of the decision under attack. Implicitly it follows that a successful petition for review results in a decision of the Enlarged Board of Appeal which sets aside the decision of the board of appeal, i.e. overturns its res judicata effect, and re-opens appeal proceedings (explanatory remarks in OJ SE 4/2007, 128). Decisions of the boards of appeal must remain final decisions (see also R 1/08).

Under no circumstances may the petition for review be a means to review the application of substantive law. This restriction is justified because the function of the petition for review is to remedy intolerable deficiencies occurring in individual appeal proceedings, not to further the development of EPO procedural practice or to ensure the uniform application of the law (explanatory remarks in OJ SE 4/2007, p. 126; R 13/10: established case law since R 1/08). The Enlarged Board has no competence under Art. 112a EPC to examine the merits of the decision and to go into the substance of a case (R 4/09, R 13/10, R 5/15), not even indirectly (R 19/11, R 6/13, R 3/18). A review of the correct application of substantive law would amount to the Enlarged Board being a third instance which has been explicitly excluded (R 3/09; see also R 13/09, R 3/18). It is not the purpose of petition for review proceedings to evaluate whether or not the reasons selected by the board are appropriate (R 13/14, R 2/18); the Enlarged Board cannot replace a board's substantive assessment with its own (R 9/14). The Enlarged Board cannot act as a third instance or second-tier appellate tribunal in petition proceedings (R 9/10, R 11/11, R 5/13; see also R 1/08, R 3/09, R 13/09, R 3/18).

Not being competent to decide on the merits of a case necessarily implies that the Enlarged Board has no power to control the normal exercise a board makes of its discretion (R 10/09, see also R 6/17). The exercise of discretion is only subject to review if arbitrary or manifestly illegal (R 10/11), thereby involving a fundamental violation of the right to be heard (R 9/11; see also R 17/11).

In R 13/12 the Enlarged Board stated that it had to remain vigilant and thwart any attempt to blur the frontier between what might clearly be a matter for a violation of the right to be heard under Art. 113 and 112a(2)(c) EPC, and anything else presented as a violation of the right to be heard but which actually pertained to the substantive merits of the decision under review.

In R 3/18 the Enlarged Board noted that the question at issue, i.e. the "estoppel situation" as characterised by the petitioner, might, in principle be eligible for a referral to the Enlarged Board under Art. 112 EPC. However, the Enlarged Board held that an absence of such a referral under Art. 112 EPC did not entitle the Enlarged Board in proceedings under Art. 112a EPC to deal with the petitioner's request to correct this estoppel situation and to set aside the decision under review.

The following issues cannot be considered in review proceedings: