Commanding a dozen regiments during the heat of battle can be a daunting task. To make it easier, you can select and group up to six regiments at the same time and give them orders simultaneously. To select and group multiple regiments, click on an open area, hold down the left mouse button and drag a box around all of the regiments you wish to include. You can also manually add or remove regiments by holding down [Shift] or [Ctrl] and clicking on a regiment.

Once you have selected a group, you can save it by holding down [Ctrl] and pressing any of the keyboard number keys. This assigns all of the regiments in that group to that number key, allowing you to reselect that group by pressing that number key. When you assign a group to a number key in this way, that group appears in the unit bar at the top of the screen. Note: You can add groups to the unit bar directly by right-clicking in an empty square, and can select them by clicking on their portraits in the bar. If you like, you can also assign individual regiments and buildings to hotkeys and the unit bar in the same way. Regiments in a group are a bunch of individual regiments grouped together.When given orders, they move and fight independently, each in its own separate formation. Slower regiments are left behind, and weaker regiments end up face-to-face with the enemy, just as if it were alone. If you want a group to act together, select them and click on the Group Behavior icon [V] to form them into a division. A division works like a single giant regiment. If placed in Assault formation, each regiment takes up that formation, and the entire division forms a single, larger Assault formation, putting weaker regiments to the rear and stronger ones to the front. A division moves and fights together, giving it an advantage over grouped units acting separately. Click on the Group Behavior icon again to can separate a division back into a group.



Source: Axis and Allies RTS Official Manual