SBML Level 3 Version 1 introduced the ability to include an attribute `sbml:units` on MathML `cn` elements appearing in SBML mathematical formulas. The value of this attribute can be used to indicate the unit of measurement to be associated with the number in the content of the `cn` element. The value of this attribute must be the identifier of a unit of measurement defined by SBML or the enclosing Model. Here, the `sbml` portion is an XML namespace prefix that must be associated with the SBML namespace for SBML Level 3. The following example illustrates how this attribute can be used to define a number with value `10` and unit of measurement `second`:

    <math xmlns="http://www.w3.org/1998/Math/MathML"
          xmlns:sbml="http://www.sbml.org/sbml/level3/version1/core">
            <cn type="integer" sbml:units="second"> 10 </cn>
    </math>