what are people refering to when they say the AC factor or value in zbrush?
I just know the AD Factor - which means “Alpha Depth” Factor and is the multiplier for the Alpha Strength at the bottom of the Alpha Menu (for example applying an alpha with “Crop and Fill” to the canvas the Alpha Depth Factor will determine the depth).
The Alpha Depth Factor gets set by ZB after Displacement Extraction to help you set the correct Displacement Scale in your 3rd party application (might not happen with the new displacement extraction in 3.5/4 – haven’t checked this in a while)
But then again there might be an AC Factor I don’t know about 
can be more specific…AC is usually Antes de Cristo:D