If a unit wins a combat and the enemies it is in contact with flee, then the victorious unit must normally pursue. Note that a unit will only pursue if all the enemies it is in contact with are fleeing - if one enemy unit breaks and flees while others fight on, the victorious troops cannot pursue.
Even if a unit can pursue, the controlling player may decide that he would prefer his unit not to pursue the fleeing enemies. Normally a unit must pursue if it is able to do so, but the player may attempt to halt the pursuit by making a test against the unit's Leadership (Ld). This represents the unit's leader calling to his troops to hold, while their natural inclination is to run after the enemy and destroy them. If the test is passed, the unit may remain stationary instead of pursuing.
A unit does not have to pursue if it is defending a wall, building, hedgerow or a comparable obstacle or fortification. As pursuit would force the unit to abandon its secure position there is obviously an incentive to stay put! In such cases, the player can automatically choose whether to pursue or not.