Jay i would interpet it this way. The stepwedge useage tells you what your materials are capable of with different exposure and development for both film and paper. When dealing with the stepwedges you are not at that time dealing with either zones or SBR. That basic data is used in either application to zone system work or incident system work. The SBR is only referenced to the incident system and does not deal whatsoever with the zone system. Go to your BTZS manual and see if you can find with the zone system the usage of the term SBR. Go to the incident system and I am certain that you will find it being used with regularity.
Of course Claire may have egg colored face if you find such a case.