Identification is normally achieved through particular pieces of information which we may call “identifiers” and which hold a particularly privileged and close relationship with the particular individual. Examples are outward signs of the appearance of this person, like height, hair colour, clothing, etc… or a quality of the person which cannot be immediately perceived, like a profession, a function, a name etc. The Directive mentions those “identifiers” in the definition of “personal data” in Article 2 when it states that a natural person "can be identified, directly or indirectly, in particular by reference to an identification number or to one or more factors specific to his physical, physiological, mental, economic, cultural or social identity"

Recital 33 refers to this sort of data as “data which are capable by their nature of infringing fundamental freedoms or privacy”. It is reasonable to think that the legislator may have felt similar concerns regarding national identification numbers due to their strong potential for easily and unequivocally connecting different pieces of information about a given individual.

— Opinion 4/2007 on the concept of personal data