This specifies the interval and unit of measurement of ordering quantities for which the Price Specification is valid. This allows e.g. specifying that a certain freight charge is valid only for a certain quantity.
Note that if an offering is a bundle, i.e. it consists of more than one unit of a single type of good, or if the unit of measurement for the good is different from unit (Common Code C62), then hasEligibleQuantity refers to units of this bundle. In other words, "C62" for "Units or pieces" is usually the appropriate Unit Of Measurement.