In the United States federal regulations require that in order to label coffee as "decaffeinated" that coffee must have had its caffeine level reduced by no less than 97.5 percent.
Example: Panamanian coffee is about 1.36% caffeine by weight normally. This and many other arabica coffees are about 98.64% caffeine free even before anything is done to lower the caffeine content..
When 97% of the caffeine has been removed only .0408 % of the coffee weight is caffeine. About 4/100ths of 1%. At this level it is labeled "decaffeinated. How roasters label their products is another matter. Suppose two roasters roast Panama coffee that originally came from the same lot, and were decaffeinated together in the same vat. One roaster labels his decaf. "97.5% Caffeine Removed." The other says his is "99+% Caffeine Free." Which roaster is not telling the truth?
The answer is: They are both right. They are both essentially saying the same thing.
Decaf should range somewhere in the 2-4 milligrams of caffeine per cup range.
Currently used solvents for decaffeinating coffee include, H2O (water), CO2 (Carbon Dioxide), Meth. Chloride, Ethyl Acetate. Note: A relatively new method called Swiss Water Decaffeinated uses "flavor-charged" water in the decaffeinating process.