The "units" attribute is defined only in SBML Level 1. In SBML Level 2 and Level 3, it has been replaced by a combination of "substanceUnits" and the units of the Compartment object in which a species is located. In SBML Level 2 Versions 1–2, an additional attribute "spatialSizeUnits" helps determine the units of the species quantity, but this attribute was removed in later versions of SBML Level 2.
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