Front/Rear/Side Armor

What about Side Armor?

The shortest answer is: There is no side armor.
However, there are "bonuses" when you fire your weapons at vehicles from the side.

The vehicles are divided into halves, the front half and the rear half.
From the sides, the chance of hitting the rear section is greatly increased.

When a vehicle is moving forward, the chance of hitting the rear armor is increased.
Caculated hits will bend space and time and often hit the rear.
Misses will scatter and are more likely to hit the rear if it didn't scatter too far.

What about Armored Skirts?

Armored Skirts are not physical plates of steel that protects the vehicles.
The upgrades themselves often carry modifiers that boosts defenses of the vehicle.
Basically, they are just for show and identification.

Front and Rear

All tanks uses a different rear armor type, they take addition penetration, and use a different set of criticals when getting hits from the rear.

Some vehicles does not carry a rear armor type. These units do not take additional penetration, they do not use a different set of criticals.
Most of these units are light vehicles, such as jeeps, bike, bren carrier, and halftracks.

Some light vehicles use rear armor, but both front and rear armor uses the same critical type. These vehicles may take addition penetration when getting shot at from the rear, but do not use a different set of criticals.
Most of these units are medium vehicles, such as armored cars, panzer elite light vehicles (non-halftrack), and the kettenrad.