Not only varies by property, but often the property varies their policy too; be sure to ask your b'fast server.

On different visits, Hyatt Sacramento offered diff usage on buffet, menu credit, Sunday Brunch credit, and even off the lunch menu as well. YMMV, but probably depends on whether the certs are a result of a closed RC or b'fast rate, if they offer buffet, if it's weekend vs. weekday vs. holiday, if they offer brunch, and even what time of day you eat. At some properties, the buffet hours differ from b'fast menu hours, and I've often been offered the menu option when the buffet is closed. Hyatt Santa Clara for instance, offered menu credit over the Christmas holidays when they were really slow, RC was closed, and buffet wasn't offered. Sonoma generally offers menu credit. Hyatt SF seems to stick to no-hot-items buffet only, if RC is closed. Hyatt SFO Airport use to allow credit toward cafe deli items, inc. sandwich and salads. Once, Grand SF was "anything from the menu" when RC was closed.

Tip: I've also often used the certs as credit on something to-go, especially if after b'fast hours, or just don't have the time, or not hungry. Usually I'd ask the host/hostess to trade the cert for a mocha, soft drink, or cup of coffee and maybe a muffin to go; think I've been turned down only once in all the years. (Key seems to be to ask for something they don't need to cook.) I frequently do this at checkout with the extra AAA certs if staying on b'fast rate, and RC was open -- something to snack on the road, especially if RC food is long gone.