A unique identifier for the Individual.
This uniquely identifies the Individual, and persists across IndividualAggregator instances. It may not persist across linking the Individual with other Individuals.
This is an opaque string and has no structure.
If an identifier is required which will be used for a long-lived link between different stored data, it may be more desirable to use the Persona.uid of the most relevant Persona in the Individual instead. For example, if storing references to Individuals who are tagged in a photo, it may be safer to store the UID of the Persona whose backend provided the photo (e.g. Facebook).
As a special case, the ID defaults to an empty string when the individual has no personas (i.e. if it’s just been constructed).