Each assigned chemical shift should map to one or more nuclei of the studied 
            biological macromolecule or complex. In preliminary reports and for some older entries, 
            this mapping is not always accurate and some chemical shifts can therefore be "unmapped". 
            It implies, that the chemical shift was parsed, but not mapped to any nucleus expected 
            from the molecular description. It may result for instance from typos or 
            from an incomplete molecular description. Details are given in the free text "diagnostic" attribute.