A specific term is needed in order to annotate variables (parameters) in SBML as the specific growth rate µ (see the definition in the paper http://identifiers.org/pubmed/24084808 and its sublement). This is a parameter that describes the time it takes to produce one gram of cellular dry weight and it has the unit per hour.
Anonymous
created term SBO:0000610 - 'growth rate'.
For now this is a direct child of 'quantitative systems description parameter (SBO:0000002), but we should refine its location in the tree...