There are two types of identifiers, i.e., direct and indirect.
Direct Identifiers
Information that relates specifically to an individual. HIPAA designates the following as direct identifiers: names; postal address information other than town or city, state, and zip code; phone numbers; fax numbers; email addresses; social security numbers; medical record numbers; health plan beneficiary numbers; account numbers; certificate/license numbers; vehicle identifiers and serial numbers including license plate numbers; device identifiers and serial numbers; URLs; IP addresses; biometric identifiers; and full face photographic images and any comparable images.
Indirect Identifiers
Information that can be combined with other information to potentially identify a specific individual. HIPAA designates the following as indirect identifiers: city, state, and zip codes; elements of dates; and other numbers, characteristics, or codes not HIPAA-designated as direct identifiers.