Register rewards (RR) are coupons from the Catalina machine that print after you make a qualifying purchase. This are used as cash would and you get as many as you want from that promotion as long as they are purchased in different transaction.
You can not use the RR you got from a promotion to buy the same one. You can buy different promotions on your same transaction and different RR will print.
RRs explicitly say that they can not be used on tobacco, alcohol, tax, lottery tickets, money orders, stamps, dairy, or any gift cards or pre-paid cards.
Walgreen's policy states that you you cannot use more RRs and manufacturer coupons than number of items you are buying. Store coupons do not affect how many manufacturer coupons and RR you are able to use .
If you are using a manufacturer's coupon and a Walgreens coupon on the same item and hoping for overage towards your other items, give the manufacturer's coupon first otherwise the coupon will be rejected by the register if it is over the cost of the item, but a wags coupon will go through; as long as the total before taxes is more or equals the amount of the coupons.