SBR stands for subject brightness range, or subject luminance range. Davis uses SBR rather than SLR so as to not to confuse with single lens reflex.
SBR is determined with an incident reading of the shadows and highlights of the subject, and provides us with an idea of the contrast range of the subject, and therfore what kind of development the film needs for the lighting conditions.
It provides the same kind of data that we get from the Zone system and can rougly be equated to N numbers as followsl.
SBR 9, = more or less N-2
SBR 8, = more or less N-1
SBR 7, = more or less N
SBR 6, = more or less N+1
SBR 5.5, = more or less N+2
I prefer the SBR method of metering to the Zone system and use it for most of my work.