Coal Resource Classification System of the U.S. Geological Survey

http://pubs.usgs.gov/circ/c891/crcs.htm

The classification system by which coal is classified into resource/reserve base/reserve categories on the basis of the geologic assurance of the existence of those categories and on the economic feasibility of their recovery.

by Wood, Gordon H.; Kehn, Thomas M.; Carter, M. Devereux; and Culbertson, William C.

U.S. Geological Survey Circular 891