Personal Identifiers (PID) are a subset of personally identifiable information (PII) data elements, which identify an individual and can permit another person to "assume" that individual's identity without their knowledge or consent.
[1] PIIs include direct identifiers (name, social security number) and indirect identifiers (race, ethnicity, age).
[2] Identifiers can be sensitive and non-sensitive, depending on whether it is a direct identifier that is uniquely associated with a person or a quasi-identifier that is not unique.
A quasi-identifier cannot pin down an individual alone - it has to be combined with other identifiers.
[3][4] Treated as PID globally, not just for citizens of the EU