The data owner's ordering of this identifier among all identifiers of the same type that they associate with the same person. 1 is the owner's primary or preferred value, 2 the next, and so on. A null value means the owner supplies no ordering, which is usually because the person has only one identifier of this type. The ranking is the supplier's own judgment and is only comparable within a single supplier's data, in the same way as identifier_quality_score (368). Use it to take the best few identifiers per person rather than all of them, for example the primary email address instead of every address ever associated with someone.