Questions about upgrades

I have always wondered even back in the SC1 days about the upgrades. I'm talking about the upgrades that you get the effect your unit's attack and defense. (engineering bay, evolution, forge.)

So upgrading attack with most units increases it's attack by 1 or more for several units. However it's the armor that confuses me, how is armor reduced? By percent, or just by 1? or what...?

Thank you very much guys
-New to the forum

 

Armour never goes down. As far as I know your armour/shields just goes up by 1 per upgrade you do.

 

The damage is reduced by the exact armor value.

Example: A Zergling with 2 armor gets hit by a Marine that deals 6 damage. Six minus two equals four damage taken.

 

ty ty