The question of allowability of amendments is legally a question of whether the application as so amended is allowable. An amended application must of course satisfy all the requirements of the EPC including, in particular, inventive step and the other matters listed in B‑XI, 3.6 (see also C‑III, 2).
If, however, the applicant seeks to amend the description (other than references to the prior art, see H‑IV, 2.2.6), the drawings or the claims in such a way that subject-matter which extends beyond the content of the application as filed is thereby introduced, the application as so amended cannot be allowed.
The underlying idea of Art. 123(2) is that an applicant is applicants are not allowed to improve his their position by adding subject-matter not disclosed in the application as filed, which would give him an unwarranted advantage and could be damaging to the legal security of third parties relying on the content of the original application (see G 1/93).
An amendment should be is regarded as introducing subject-matter which extends beyond the content of the application as filed, and therefore unallowable, if the overall change in the content of the application (whether by way of addition, alteration or excision) results in the skilled person being presented with information which is not directly and unambiguously derivable from that previously presented by the application, even when account is taken of matter which is implicit to a person skilled in the art (see G 2/10).