From Elsmar Cove Quality Assurance and Business Standards Wiki
Machine Capability - Often Known as Cmk. This is a short term machine capability index derived from the observations from uninterrupted production run. Even though the formulae are as same as Cp & Cpk calculation, the Standard Deviation used here is sample standard deviation (RMS). The preferred Cmk value is >1.67. Usually, The long term process capability to be planned and studied after achieving the targeted Cmk Value (Venkat).
aka Cmk
Elsmar Cove forum Machine Capability discussion threads