Drill Manuals is a helpful policy to adopt from the Renaissance to early Modern Eras, during which you'll need Niter to upgrade your army with Musketmen and Bombards and Coal to upgrade your navy with Ironclads and Battleships.
Improvements to the reliability and manufacture of firearms during the Renaissance placed these weapons in the hands of increasingly large numbers of soldiers. Coupling this with the rise of the nation-state and its ability to conscript subjects to serve in the army, and generals found themselves with a new problem: The need to convey specific, technical knowledge to largely ignorant masses of conscripts. To this, the drill manual was developed. Various nations and authors wrote drill manuals between the 16th and 19th centuries, with the practice falling off with the advent of more modern firearms.
The drill manual is intended for line officers and non-commissioned officers to teach the basics of maneuver and fire to non-specialists. It emphasizes the rote learning of commands to the point where they can be done automatically by the soldiery, freeing up the officers to consider the tactical and strategic situation on the battlefield.