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Rather than being a simple measure of defence, it primarily relates to how much damage the shield's durability rating takes from a single hit. A Shield Guard higher than the enemy's weapon rating will greatly reduce the damage the shield takes while blocking, while the reverse can cause a shield to break much faster, even with the same durability.
It also relates to how an enemy will react when Link blocks their attack with a shield - a sufficiently high Shield Guard will cause the enemy to stagger or even be disarmed.