For weapons with an additional effect that lasts some duration (eg: def down on magian claws), the add.effect can't proc if it's already on the mob. Therefore to figure out the actual proc rate, it only counts the number of hits that are outside a 30 second window after a proc as "restricted' hits -- hits that can be considered valid for purposes of a possible proc.
If that doesn't apply for the weapon you're examining, you can ignore that part.
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