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Reference mapping rules specify the construction of xfMap reference-sets, and they must be defined inside the <reference-map> or <reference-content-map> elements.
References allow an xfMap to store values that need to be accessed later in the input stream. All reference mapping rules, whether they’re defined under the <reference-map> or the <reference-content-map> elements, have the same structure. Similar to group mapping rules but unlike feature mapping rules, every reference mapping rule when activated, regardless of where it was defined, triggers the XML Reader to construct a new reference-set.