1. A method of specifying individual functional window units in a modular fenestration system that includes a collection of individual functional window units, the collection comprising functional window units of different configurations with each configuration comprising functional window units of different sizes, each respective configuration of the functional window units of different configurations having at least one physical attribute that is common to the functional window units of other configurations of the functional window units of different configurations, the method comprising the steps of establishing an array of roughed-in opening sizes that vary by a predetermined increment, sizing each of the functional window units of the collection to fit within a corresponding one of the roughed-in opening sizes of the established array, wherein the size of each functional window unit is different than the corresponding roughed-in opening size, and specifying each functional window unit of the collection by the roughed-in opening size in which it is sized to fit.
2. The method of claim 1 , and further comprising specifying each window unit of the system by its configuration.