A scoped, opaque, not reassignable, targeted pseudonym for a subject (person) – see Name Identifiers for terminology.
https://wiki.oasis-open.org/security/SAMLSubjectIDAttr

Intended to replace legacy identifiers such as eduPersonTargetedID and SAML 2.0 persistent NameIDs due to their case-folding issues as well as deployment complexities.

Examples: