What is a "partial identity" (Teil-Identität), and why does every organisation only ever hold one?

A partial identity is the slice of a subject's properties that one organisation records in its context; since no system needs or should hold everything about a person, each registration creates a partial identity, and authentication checks that this partial identity belongs to the subject.

* One person, several partial identities: each organisation records only the slice it needs. *

A person has an unbounded set of properties. A university needs name, matriculation number and programme; a shop needs delivery address and payment method; a gym needs a photo and membership status. Each of them registers a partial identity, and:

• Data minimisation is built in: nobody needs your gym membership to sell you a train ticket.
• Several partial identities coexist for one subject and can be linked when useful.
• Authentication is defined per partial identity: it verifies that the identity presented in this context belongs to the subject, using the authentication means bound in this context.

The privacy debate around central identity providers is essentially about who gets to see across partial identities; a login provider involved in every context can join them into a profile, which is what user-centric and self-sovereign models try to prevent.

Go deeper:

• Pfitzmann and Hansen, A terminology for talking about privacy by data minimization (PDF) — the paper that defined partial identity, pseudonymity and unlinkability.
• Data minimization (Wikipedia) — the GDPR principle that partial identities implement.