Binding Strength is the power with which a spell is bound to an enchanted item or spell scroll.
When casting a spell using an enchanted item or spell scroll, the binding strength is the level at which the spell is cast, and the caster counts as having passed the casting test with a DoS equal to the Binding Strength.
When creating an enchanted item or spell scroll, the Dos of the test to create the item becomes the items Binding Strength.