This is a very useful tip for thining out the opposing army's troops, when an enemy officier is in a "blue" area and you kill him, he will leave for the rest of the game, and so will all their troops. Example, if there are 3 strongholds and the 2 outer ones are blue and the officier is somewhere in between them, he is on the "blue area". The same thing applies for your officiers, if they are in a red area and are killed they will leave for the rest of the game.