Ed - SBR = Subject Brightness Range. Careful zone system practitioners will document the SBR (readings from shadows and highlights etc) for each sheet exposed then adjust development times to ensure all negatives will print on a given paper grade with full tonal rendering - eg extend dev times for subjects with short contrast ranges and reduce times for subjects with high contrast.
This was an important thing to do in the days before multi grade papers were as good as they are now. I've never seen the point with modern materials, good compensating developers and a little bit of darkroom skill. That said, there's nothing wrong with working in this way.