Basically it means for the Dirham rule to apply, it has to be blood itself. If it’s water mixed with blood, or moisture that blood made najes and this moisture got on your body or clothes, then the Dirham rule doesn’t apply. Another example is let’s say milk. This milk became najes because of blood. If less than a dirham of that milk comes on your body, you cannot pray with it. The Dirham rule doesn’t apply here.