NDF-RT content model and the example. Triangles denote hierarchies of related concepts, categorized in the rectangles within the triangles. Taxonomic or ISA relationships (upward-pointing blue solid arrows) unify NDF-RT™ clinical drug concepts into a polyhierarchy, classified both by their VA drug class and their generic ingredient(s). Various named role relationships (sideways-pointing amber dash arrows) define the central drug concepts (green) from which they originate in terms of the reference hierarchy concepts (blue) pointed to. Role relationships are also inherited into subsumed clinical drug concepts. Adapted from NDF-RT document [April 2012 Version] .