Article 4(5) of Regulation (EU) No 376/2014 states that "The Commission shall, by means of implementing acts, adopt a list classifying occurrences to be referred to when reporting occurrences..."
This list is provided in Commission Implementing Regulation (EU) 2015/1018 of 29 June 2015 laying down a list classifying occurrences in civil aviation to be mandatorily reported according to Regulation (EU) No 376/2014 of the European Parliament and of the Council.