An identifier token, or null — the value a "which registered thing backs this" field carries. null is the value that says NOTHING is named; the empty string is refused, so the two can never be confused on the wire.
null
An identifier token, or
null— the value a "which registered thing backs this" field carries.nullis the value that says NOTHING is named; the empty string is refused, so the two can never be confused on the wire.