cold side-compressor side, the larger the AR obviously the more air and CFM it will move, the only drawback is air will only move so quickly at a given speed and time.

hot side-exhuast turbine, the ARs usually seen are .48, .63, .81 these are just like on the cold side except they depict on how fast exhaust will escape, also directly relating to the size of the housing, they also have stages I, II, III wheels, these by some turbo companies are either "clipped" or angle change sin the blades to allow a little more exhuast flow from a smaller exhaust housing.

general rule, a smaller AR will give great low and mid boost response like say a .48 on a sohcD16, and will be perfect for its power beand and exhuast volume, put that same turbo on a ITR and it will not be able to "breathe" at the higher rpms and kinda choke out power. a .63 would be best suited most of the time for a B series.

hope this helps

a good turbo for a sohc, T3 super 60!