Infantry (eg: Soldiers, robots, etc)
Vehicle (eg: tanks, etc)
Aircraft (eg: fighters/bombers/dropships)
-SNIP- I'll talk about space later
So each one of these unit classes could have a category of armour:-
-SNIP- maybe more
Then each of these armour types could have a material:-
And so forth.
So to describe an armour, you have to describe a) what's wearing it (and what CAN wear it), b) how 'heavy' it is (eg: light) and c) what it's made of. All of these different classes/categories provide a set of attributes...
So "Infantry/Light/Cloth" armour would create a specific defense score, a specific speed score, and also provide a set of additional attributes/weaknesses against certain weapons.
When we get to weapons, we have a similar system:- "Bolter" is defined as "Infantry/Projectile/Explosive" (as an example), each of which would then give the weapon a certain attack score, with the 'Explosive' attribute perhaps granting additional bonuses when used against 'Cloth' armour and so forth.
With this system in mind, we'd perhaps build the scores up as a like-to-like comparison of each element?
Attack - Defend
Infantry vs Infantry (no bonus)
Projectile vs Light (+1 attack or -1 defense - perhaps due to projectiles easily penetrating armour, whatever)
Explosive vs Cloth (+3 attack or -3 defense - due to cloth being useless next to a big bang)
In this case, we'd see an accumulation of +4 attack (or -4 defense, whatever) due to the various attributes stacking.
This system would work in the case of "Man with handgun attacking huge tank" and "Man with rocket launcher attacking tank"
Attack - Defend ("Man with handgun attacking huge tank")
Infantry vs Large Vehicle (+4 defense)
Projectile vs Heavy (+3 defense)
Non-Explosive vs Composite Alloy (+5 defense)
Attack - Defend ("Man with rocket launcher attacking tank")
Infantry vs Vehicle (+2 def)
Projectile vs Heavy (+3 def)
High Explosive vs Composite Alloy (+5 attack)
It's an idea anyway.