Armour values are different for each type of vehicle. They represent not just the thickness of its armour but also how difficult a target it is because of its size and speed, how tough and numerous its crew are, etc.
Armour values for individual vehicles also vary depending on whether the shot comes from the front, sides or rear.
When a vehicle is hit by a non-barrage weapon with a Blast marker or large Blast marker (including ordnance), the shot is always assumed to strike the vehicle from the direction of the firer regardless of exactly where the marker is placed or scattered to.
In the case of a barrage weapon, the shot is assumed to come from the hole at the centre of the Blast marker. If this is over the vehicle then it is struck from the direction of the firer, if it is behind the vehicle then the shot hits the rear armour, and so on. Remember the rule about the hole in the middle of the Blast marker! A barrage shot landing behind a vehicle may hit the rear armour but will be at half Strength.
Diagram: Vehicle facings for armour purposes, viewed from directly above a tank. Diagonal dashed lines from each corner divide the vehicle into four facings: Front (Armour Value 13), Side (Armour Value 11) on both left and right, and Rear (Armour Value 10).