-B parameter is documented as:
Get/set Advanced Power Management feature, if the drive supports it. A low value means aggressive power management and a high value means better per‐ formance. Possible settings range from values 1 through 127 (which permit spin-down), and values 128 through 254 (which do not permit spin-down). The highest degree of power management is attained with a setting of 1, and the highest I/O performance with a setting of 254. A value of 255 tells hdparm to disable Advanced Power Management altogether on the drive (not all drives support disabling it, but most do).
This only gives two possible intervals, but doesn't describe what other effect different values have. What's the difference between let's say 63 and 127, etc. I couldn't find any more documentation about this. Is it described somewhere?