What are the requirements for advanced electronic signatures?

The requirements for advanced electronic signatures are (Article 26 of Regulation 910/2014):
An advanced electronic signature shall meet the following requirements:
(a) it is uniquely linked to the signatory;
(b) it is capable of identifying the signatory;
(c) it is created using electronic signature creation data that the signatory can, with a high level of confidence, use under his sole control; and
(d) it is linked to the data signed therewith in such a way that any subsequent change in the data is detectable.

[GMP Compliance Adviser, Chapter 15.A]