While your army can function effectively for most battles with the default unit categories, you will certainly find it more useful to increase the control that you have over specific types of troops by separating them as much as possible.
Without your own customisation, troops will be assigned into four basic categories: Infantry, Ranged, Melee Cavalry, and Ranged Cavalry.
Once you have enough of each type, you can then consider how you might want to assign them more specifically, such as separating bows from crossbows, or assigning throwing units to their own key so that you can use them to flank an enemy and function as ranged shock troops. For example, if you have multiple ranged units of different cultures, skill level, or even weapon type (between bows, crossbows, and throwing weapons), these will be assigned to the same number as ranged units by default. The particular troops which are assigned to each number by default will depend on the structure of your army. You can also change which keys activate specific orders and formations from the same options menu.
You can change the number which specific groups of units are assigned to from the Order Menu in Mouse and Keyboard options in game.
