Instead of an AR, shields have a Block Rating (BR), which reflects a combination of the shield’s coverage and toughness. If a character blocks an attack with a shield, then the attacker rolls damage with the attacking weapon. If the damage result is less than or equal to the shield’s BR, nothing happens. If the damage result exceeds the shield’s BR, then the character takes that damage on their shield arm. Shields count their BR as being half as much (round up) against attacks that deal magic damage unless they have a unique BR listed for magical attacks.
This reflects the way that, while a shield can protect its user from harm, using a shield to block repeated heavy hits can quickly drain a defender’s energy even if they are still able to avoid direct harm. Eventually they will find themselves tiring and unable to maintain their defense.