To the best of my knowledge, we don´t know for sure. The part with the shield protecting the front and left I´ve seen quite often, so that´s probably correct. The defense, I think affects only melee combat, while armor and shield protect from missiles as well.

As to how the values work, I *think* they represent virtual dices, much like in a P&P roleplaygame. Each dice (one point of attack/defense) has a certain chance for sucess, either to score a lethal hit (since most units have only one point of "Life energy") or to block a hit. Mind you, in combat, it´s not the unit as a whole that is fighting, it´s always the individual men in it (which, being clones, have all the same statistics)