Using different doors to get in and out of buildings is one of the most underutilized abilities in the game, and yet it's so easy and requires very little micro for the potential gain. Countless squads have been saved by picking the door they get out of so they can retreat away with low health. Getting a squad out of the building at the door away from enemy fire will save the squad from taking extra fire and possibly losing the squad. You can even use this option as a shortcut or to avoid wire.
The Basics
To control the door your units use to enter the building you must get your squad close to the door you want to use; then command your unit to garrison the building they will then use the door closest to them.
To control the door your units exit from you must select the unit inside the building and hit "D" or the unload icon and then left click on the side of the building you want your unit to exit from. This will only work if there is a door on that side. If there is no door on that side your units will exit the building using another door and rally at that point.
Not all buildings have multiple doors, like this building near the center of rails and metal.
Note the back side of the building has no doors.
Larger buildings may have doors on all four sides, like this behemoth on Sturzdorf.
Backside of the same building.
Using multiple doors can be used as a shortcut, or to bypass defenses or obstacles. Seen here a shortcut on northern Lorraine.
Backside view of the same houses.
A high level trick is to throw a grenade at the door of a building, so if the units exit through the default door they will have a nasty surprise. Here two Riflemen have thrown Grenades at both the building and the door in an attempt to slaughter the Volks, who have wisely decided to use the back door instead.
This tip of the week was produced cooperatively by ntd, dcw, Joe, and Siou.