What is the difference between assessed “resources” and assessed “reserves”? Why doesn’t the USGS assess both resource and reserves?

"RESERVES" are those quantities of petroleum anticipated to be commercially recoverable by application of development projects to known accumulations from a given date forward under defined conditions. Reserves must further satisfy four criteria: they must be discovered, recoverable, commercial, and remaining (as of the evaluation date) based on the development project(s) applied" (page 213).

The USGS uses UNDISCOVERED RESOURCES to encompass all quantities of petroleum (recoverable and unrecoverable) naturally occurring on or within the Earth's crust that have not been discovered. It includes all types of petroleum whether it be conventional or unconventional.

The USGS does not assess reserves, because in order to do so, it would have to have knowledge of the financial and revenue structure of those companies holding previously discovered resources as well as knowledge of their geological picture and engineering capabilities to determine if the discovered resources meet the four criteria above.