This has been discussed many times... generally on voluntary, they can offer either a credit (common on International Init) or a voucher (more common on domestic). In the case of massively overbooked flights, there's some haggling that can be done--the longer you wait, the less chance of a voucher/credit, but the more it'll be (mini-reverse dutch auction). At some point, they'll just decide to IDB, and then people are SOL under the federal guidelines (in some cases, you'll get nothing at all under IDB).

What you take/use is up to you. If you're a mile whore (who? me?), or, even more importantly, an EQM whore (WHO? WE?), you'll usually take the voucher. It's handy, it's got a cash value, it's EQM and mile earning and can be used to buy tickets in specific fare ratings. If you're not a mile whore, you wouldn't be reading this. But, for argument's sake, let's say you were and you stumbled into this thread, and you're patient, the voucher could get you a nice, extremely restricted ticket. You enjoy that.

Timothy

Vouchers! Vouchers! Vouchers!