"Effective Against Armor"

[Deus ret.]

I guess that is what it is supposed to represent, but in the melee-combat calculation defence and armour are added up to the total defence value, which the determines the odds of a strike being successful or not. If a strike is successful, the target dies (unless he happens to have multiple hitpoints, but most soldiers have only one). However, like you say, only armour protects against missiles.

Armour protects vs missiles. Defence vs melee. They do not add together, though 'armour bonuses' are added to both armour and defence.

If this is correct how exactly would the armour piercing bonus work? I found match-ups like the BM-CK described by libra683 to be quite in favour of the armour piercing unit, nonwithstanding a possible high defense rating of the -in this case- mounted victim. It seems like the units' defense rating doesn't really count any more once they wear heavy armour and are attacked by an armour piercing unit.