An EPD is an estimate of how future progeny of a particular sire are expected to perform, compared to progeny from other parents, when mated to comparable cows and the progeny are treated similarly. EPDs are calculated on traits which can be measured quantitatively and are always expressed in units of measurement.
Units of measurement can be pounds, inches, square inches, degrees, centimeters, etc.
Each individual expresses itself through a phenotype. A phenotype is the result of both genotypic and environmental influences exerted on an animal. This is the fraction of genotypic variance that is separated out in variance component estimation, and used to calculate heritability for a particular trait. It must be remembered that this is strictly a simple formula and EPDs are actually calculated using much more advanced mathematical procedures, along with much more genetic information. Genetic information includes heritability estimates, genetic correlations, genotypic and phenotypic variances and covariances, and degree of relationship between individuals.