The following excerpts and definitions from U.S. Geological Survey Circular 831, Principles of a Resource/Reserve Classification for Minerals (www.ut.blm.gov/NewsReleases/nrjan16USGSCircular831.pdf) provides some insight into the basic concepts of classification of economic mineral resources.
Resource: A concentration of naturally occurring solid, liquid, or gaseous material in or on the Earth's crust in such form and amount that economic extraction of a commodity from the concentration is currently or potentially feasible.
Identified Resource: Resource whose location, grade, quality, and quantity [is] known or estimated from specific geologic evidence.
Reserve Base: That part of an identified resource that meets specified minimum physical and chemical criteria related to current mining and production practices, including those for grade, quality, thickness, and depth.
Reserves: That part of the reserve base which could be economically extracted or produced at the time of determination. The term...need not signify that extraction facilities are in place and operative. Reserves include only recoverable materials...
Economic Resources: This term implies that profitable extraction or production under defined investment assumptions has been established, analytically demonstrated, or assumed with reasonable certainty.