Configurational and thermal entropy yield identical numerical values for [delta]S only when the system's "dimensionless" energy gaps ([[delta] [epsilon]]/kT ) between the accessible quantized energy levels are minimized by temperature to nearly infinitesimal values so that the spreading of energy among the system's microstates becomes effectively classical and equiprobable. The molecular partition function provides the numerical value for the effective number of both accessible states and spatial configurations per molecule, for which this condition is valid at a given temperature. Considering the phenomenon of mixing and standard molar entropy values leads to the conclusion that configurational entropy calculations are significant and thermodynamically valid because of their fundamental connection to the process of random energy re-distributions in a system, via the available modes of molecular motion. (Contains 2 figures and 5 notes.)