National identification, sent as a JSON-encoded string. The server normalizes input into a compact JSON with these optional keys: nationalIdentifier, nationalIdentifierType (e.g. LEIX for an LEI), countryOfIssue (ISO 3166-1 alpha-2), registrationAuthority. If the input is not a JSON object, it is wrapped as {"nationalIdentifier":"<value>"}; if the value matches the LEI format, nationalIdentifierType is set to LEIX automatically and countryOfIssue defaults to this request's countryOfRegistration if not provided. The compacted JSON must be 240 characters or fewer. Omitting this field leaves the stored value unchanged; setting it to null clears it. On read, the value is returned exactly as stored.