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Armor

The game has 4 armor types used by the various units and one special one for buildings.

Unarmored

Light

Medium

Heavy

Building

Every unit weapon deals a specific amount of damage to each armor type. So always be mindful of the armor type of your enemy and which units can best counter it. You can see the armor class as well as the armor class its most efficient against by hovering over a unit. Note that units are always effective against the shown armor class and often also all armor classes below this.
The left icon is the armor of the unit and the right one is the armor it is classified as efficient against.

As a bonus note:

Most mechs also have a weak spot on their armor where they take extra damage. Usually this is on the back of a mech so try to take advantage of that. Due to those weak spots, mechs should always move in reverse when falling back. This is done by holding C and right clicking, till they are away from the enemy fire radius.