Priority & Processing
As explained in the Discounts & Offers article, discounts can be given a priority with 1 being the highest. Discounts are then processed from the top down, starting with the discount with the highest priority (Figure 1.1). Discounts without a priority are processed after discounts with a priority.
You can break out of processing by checking the Stop further rules processing checkbox in the general section of a discount.
Please note that order discounts and product discounts are processed separately - so if you have 1 order discount and 1 product discount, and set one of them to Stop further processing it will have no effect. If you have 2 order discounts or 2 product discounts that both apply, and you enable stop further rules processing on the highest rating, it will have an effect.
If you have 1 product discount that is in effect and also have 1 order discount that is in effect, but you do not want to give an order discounts on products already discounted, use the Only apply to products without a discount option on the order discount.