The 'quantitative parameter' branch of SBO contains 2 very similar terms: 'biochemical parameter' (SBO:0000256) and 'physical
characteristic' (SBO:0000255), where one (256) is dependent on system properties, and the other independent of life's influence (255). This division is both blurred, and irrelevant to how the terms are used in Systems Biology.
Both terms will be made obsolete. The appropriate updates to the SBML spec have been made my Mike Hucka in anticipation of this change, and references to those terms should be redirected to the parent term ('quantitative parameter', SBO:0000002). An update request has been made for the SBGN spec that references 'physical characteristic' (in Activity Flow, see tracker ID 3068940) here: