The Work Index is a fitted coefficient to Bond’s equation given in the picture above. The work index can either be measured in the laboratory (the Bond ball mill work index determination is a common example) or it can be calculated from the operating performance of a milling circuit (which would be an operating work index).
https://www.linkedin.com/pulse/operating-work-index-specific-energy-consumption-alex-doll/
Operating work index is not the specific energy consumption