This specifies the interval and unit of measurement of ordering quantities for which the gr:Offering or gr:PriceSpecification 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 gr:hasEligibleQuantity refers to units of this bundle. In other words, "C62" for "Units or pieces" is usually the appropriate unit of measurement.