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>