This is the ValueSet that contains all the codes in EntityClassChemicalSubstance (http://terminology.hl7.org/ValueSet/v3-EntityClassChemicalSubstance) and EntityClassChemicalSubstance (http://terminology.hl7.org/ValueSet/v3-EntityClassChemicalSubstance). E.g. what you have to deal with if you get resources containing codes in either of them