Sometimes, your unit will be able to charge into the flank or rear of an enemy unit. This is particularly beneficial because an attack from an unexpected direction gives you an advantage in combat.
A charging unit's position when the charge is declared determines whether it charges into the front, flank or rear of the enemy unit. A unit's position in relation to its potential charge target is determined when the charge is declared.
If the charging unit is in the charge target's front arc when the charge is declared, it charges into that unit's front. As units generally begin the game facing each other, this is the most common situation that will arise.
However, if the charging unit is within a flank arc of its charge target when the charge is declared, it charges into that unit's flank. If the charging unit is within the rear arc of its charge target when the charge is declared, it charges into that unit's rear.