Quantities from split cart lines are combined when they represent the same product or variation, including lines split by personalization data. Different variations are evaluated separately. Every line in a qualifying group receives the percentage reduction from its regular price. For sale products, the customer pays whichever is lower: the sale price or the discounted regular price.
The coupon is exclusive. Applying it removes existing coupons; applying another coupon while it is active removes this coupon. A cart notice explains the replacement.