Roger Head writes:

http://www.cleandryair.com/scfm_vs__icfm_vs__acfm.htm

This page correctly explains that CFM, SCFM, ACFM, etc., ALL refer to "the
volume of air that is compressed each minute and it is measured on the
_inlet_ side of the compressor." The "S" or "A" prefixes simply further
specify the temp and humidity of this inlet air.