I was thinking about these armor/damage types:
Light: -piercing +crushing +slashing -lightning
light ranged units, like handgunners and archers
Medium: +piercing -crushing light
melee units, some ranged and special units, ex. halberdier, outrider, alchemist
Heavy: -piercing -fire +lighting
heavy melee or short ranged units, like knights
Fortified: +fire +crushing -piercing -lighting -slashing
buildings and heavy endgame units like, treants and steam tanks
Yes, I used only 4 for now, I think that the max is 8, but we keep other ones for a reserve for now. Anyway for the damage types I try to retain the icons' meaning, so it's like this: slashing-normal, crushing-siege, fire-chaos, lightning/cold-magic I now remembered also the hero attack type, but we will keep the reserve for it now too.