OptionalextensionOptionalhalfHalf life - for a non-natural nuclide
OptionalidUnique id for inter-element referencing
OptionalidentifierSubstance identifier for each non-natural or radioisotope
OptionalmodifierExtensions that cannot be ignored even if unrecognized
OptionalmolecularThe molecular weight or weight range (for proteins, polymers or nucleic acids)
OptionalnameSubstance name for each non-natural or radioisotope
OptionalsubstitutionThe type of isotopic substitution present in a single substance
Additional content defined by implementations