Beers made from high protein grain have a smaller period of peak freshness and taste. Screenings primarily determine the level of 'maltable' grain - the more grain that passes through a screen, the less grain available to make malt. The export standard for Stirling and Harrington grain is a 2.5 mm slotted screen. The export standard for Franklin grain is a 2.2 mm slotted screen. Discounts for high screenings reflect the need to purchase more grain to make the same amount of malt.
Grain size is determined by the amount of starch deposited in the grain.
High screenings mean smaller grain and less starch. Less starch is associated with lower quality. Grain size is also important in how well the grain physically processes in the malt house. A low variation in grain size improves the processing of the grain in the malt house. Small grains are easier to wet up than large grains.