Datum Diameter vs. Pitch Diameter

[bemasamoje]

I'm a bit confused as to the difference between these two? Can someone explain the difference? Also it would be nice to know which one to use to calculate ratios as they seem to give slightly different answers given identical sheaves.

I found this link below but it hasn't been much use other than i can use it appears to sometimes give the right value for a given sheave. Maybe it just applies to Gates? I doubt it.