A Person entity is an identified entity that has interacted with or been loaded into the Mobile Database. It is identified by a PersonID, which uniquely identifies the record. A customer can choose to define an alternate_person_id, which would be a unique identify from the customer's system for reference and cross-linking.
A person can also be tracked/linked by a channel identifier, like a Phone Number (MDN). Note that these alternate identifiers are usually sufficient but are not an ideal identification as Phone Numbers can change over time.