Stacking is on by default. A 20% catalog sale plus a 10% coupon gives both. Use Stop other rules or conditions to prevent it.
Percentage coupons without a cap can give large discounts on big orders. Set Max Discount Cap.
Usage limits count paid orders. Uses are counted when orders are placed, and given back if the order is cancelled or refunded.
COD and after-order rewards. Coupons for next time and customer-group upgrades are given after payment, so for cash-on-delivery orders that's when the order is marked paid.
Guests, "Once per customer" and "First order only". A guest is recognised by the email they enter at checkout, so these rules and the Per Customer Limit work for guests too: the same email (in any capitals) counts as the same customer, whether they ordered as a guest or signed in. Until a guest has entered their email, a promotion with Once per customer or First order only does not apply yet; it applies at checkout once the email is in.
Dates use your store's time zone. Set Valid Until to the end of the last day you mean.
Inactive or expired promotions stop applying straight away, including in carts already open.
GST. Discounts reduce the taxable price of the items, so GST is charged on the discounted amount.
Coupons generated for customers (the Generate coupon bonus or Auto Coupons) are single codes; manage them under Auto Coupons.
