In T 331/87 (OJ 1991, 22) the board, building on T 260/85 (OJ 1989, 105) set out a three-point test. The board held that the replacement or removal of a feature from a claim might not be in breach of Art. 123(2) EPC 1973 if the skilled person would directly and unambiguously recognise that (1) the feature was not explained as essential in the disclosure, (2) it was not, as such, indispensable for the function of the invention in the light of the technical problem it served to solve, and (3) the replacement or removal required no real modification of other features to compensate for the change. The board also observed that the feature in question might be inessential even if it was incidentally but consistently presented in combination with other features of the invention (frequently applied until 2012, see e.g. T 708/07, T 775/07, T 2359/09, T 747/10; recently applied in T 1906/12). This test is sometimes referred to as the essentiality test (see e.g. T 2359/09, T 2599/12, T 2489/13). Note that the Guidelines H‑V, 3.1 – November 2018 version now contain a modified three-point test.
In T 404/03 the board considered that the decisions of the boards of appeal describing and using the current palette of "tests" in connection with various types of amendments might lead to some confusion. As far as the disclosure of a group of features was concerned, as distinguished from their scope, the generalisation of a feature in a claim and the isolation of features from embodiments in the description essentially both involved a deletion of a feature, namely the specific feature and the remaining features of the embodiment respectively. The board therefore considered that these cases could be subject to the same criteria as a pure deletion, and hence, in principle, the three-point test. Rather than requiring a specific statement or suggestion in the original disclosure, the three-point test was more generous to the applicant because it essentially allowed the deletion of a feature if the skilled person realised from the common general knowledge in that field that the feature had nothing to do with the invention. Moreover, in the board's view, the cases of deletion of features and addition of features had to be distinguished with respect to the allowability of amendments under Art. 123(2) EPC 1973, since the former only removed elements that were originally disclosed and hence might be judged by a skilled person to be inessential to the invention, whereas the latter added new elements lacking any basis whatsoever in the original disclosure.
In T 2300/12 the board held that the purpose of the essentiality test was to ascertain whether the replacement or removal of a feature in an originally filed independent claim resulted in an extension beyond the content of the application as originally filed. In the case at issue, however, the granted claim was for a process, whereas there had been no such process claim in the set of claims originally filed. For that reason alone, the essentiality test could not be directly applied to establish whether there had been such an inadmissible extension.
In T 2100/14 the board held that the first criterion of the three-point essentiality test was not met. Although the description did not mention features A and B explicitly as being essential, the person skilled in the art directly and unambiguously recognised that the balcony structure disclosed in the application required a corrugated plate as defined by features A and B. The mere fact that features A and B were specified in the preamble portion of the independent claims did not mean that the presence of a corrugated plate was not mandatory for the invention. A feature of the preamble portion of an independent claim was part of the definition of the claimed subject-matter which was deemed to solve the technical problem assessed in the application. Therefore, features A and B, relating to the corrugated plate, contributed to the definition of the invention as originally filed and were thus presented as essential in the application as originally filed.