A practical distinction between coal resources and coal reserves is that reserves are quantities that can be economically recovered at the time of determination using currently acceptable mining methods and available equipment.
There are limits to the flexibility of methods and equipment to adapt to variable physical conditions of coal occurrence beyond which mining becomes uneconomic.
In most instances, the two most important factors affecting recoverability of coal by either surface or underground methods are seam thickness and total depth of overburden. In some cases, shallow overburden depths and thick seams adversely affect over-all recoverability.
Other factors are strength and lithology of roof and floor strata, attitude of coal beds, prevalence of ground water, proximity of other coal beds or mine workings, access to seams, geometry of deposits, and topographic considerations.
Regulatory restrictions resulting from the occurrence of an adverse natural condition must be considered in evaluating the net effect of the condition on resource recoverability.