It's like any form of payment, with a few caveats:
1) It splits up the "payment" across all items on the ticket, if more than one, splitting up largest percentage of the "payment" onto the highest priced item, and downards from there.
2) If you return the item that has a Reward Zone cert on it, and not do any type of exchange, it'll go back to your account, and pretty much be unsuable for 30-60 days.
3) They expire after 3 months or so, so make sure you don't let them be worthless.
4) Doing a second return/exchange on the first return from an original transaction gets a litle funny, as I saw an issue a customer service rep had with a guy who was doing a second exchange/return off of a first return/exchange.
If you use a GGC and a RZ cert, the GGC always has to go first, because it's a coupon providing a discount off the item's price, not paying for the item. The RZ cert is "payment" and needs to be handed over at the end of the transaction. GGC's and RZ's work together, I use both all the time on the same transaction.