A comparison scale for how easily a material cuts, normalised so AISI 1112 steel is 100%.
The rating folds chip formation, surface finish, tool wear and power consumption into one number. It is a shop-floor comparison, not a physical property, and published values differ by roughly ±10% because each was measured under different conditions.
A high rating does not mean soft: gray cast iron rates 70% and cuts easily because the graphite breaks chips, while pure copper rates 20% and is soft but gummy. Use the rating to compare two candidate materials, not to predict a cutting speed.
Using the rating to estimate a cutting speed. It ranks materials; it does not give you an RPM.
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