Hitpoints - When the shield is hit, it loses hit points. When hit points reach 0, the shield breaks. All damage types deal the same amount of damage. Weapons with bonus vs. shields deal double raw damage.
Armor - Subtract this number from the damage that would be dealt to the shield. If this would make the damage negative, instead reduce it to 0. If a projectile hits your shield while it is on your back, add shield armor + 10 to your body armor for the hit.
I don't know how/if this works for the 0 slot shields that go on your belt.
Other notes - If a crossbow(or ballista) projectile deals over 30 + 3*(shield's armor) (in this case, 72 damage for +0 heavy board shield), it can pierce the shield and hit you anyway for reduced damage.
High armor, low hitpoint shields get a huge boost in effectiveness when loomed relative to low armor, high hitpoint shields, since the bonus is always +3 speed, +4 armor, +53 hitpoints at +3.
