Client-owned (system, identifier) pairs for this event. Omitted = leave untouched; an explicit list (including [] or null) replaces the full set. Rejected with a GraphQL error, and no partial write, when: an invalid system URL is given; an identifier is blank/whitespace or over 255 characters; a (system, identifier) pair is already claimed by another record of the same type in the organization; or two pairs in this payload normalize to the same system. See ExternalClientIdentifierInput.
