Answer:

“Actual cc/m” or “accm” is the volumetric flow measured at working conditions (at test pressure.) The actual volume of the gas is compressed by the ratio of the absolute test pressure verses standard pressure (14.696 psia). When stating flow using this terminology, it is imperative that the test pressure and temperature be stated with the flow value. In other words, a 5 accm flow varies with every different pressure and temperature when comparing it to standard conditions.
“Std cc/m” or “sccm” is the volumetric flow of gas corrected to standard atmospheric conditions (14.696 psia, 293 K (20 C)). Volumetric flow with this designator “std” is always adjusted to the standard pressure and temperature conditions above. Therefore, a 5 sccm leak rate is the same flow at any pressure.
“cc/m” stands for cubic centimeter per minute. It does not reference standard or actual conditions so it is an abbreviation for either one. Historically the leak test industry and most every other industry has universally embraced stating flow in terms of standard conditions because it eliminates the need to stipulate pressure and temperature. Because most everyone refers to leak rates in “sccm”, they usually abbreviate it to “ccm”. Today some individuals are using “ccm” to refer to “accm”. Because of this, it is very important to clarify how this flow is defined – at standard conditions or at a stated pressure and temperature.