Most future flight credits are non-transferable, meaning they can only be used by the passenger they were issued to. Travel certificates, meanwhile, can be used to book travel for anyone.
Companion Vouchers are non extendable and non transferable. Once travel using the Companion Voucher has been booked no name changes are allowed. If travel using a Companion Voucher is cancelled within 24 hours of travel then the Companion Voucher will not be re-issued to the Cardmembers Executive Club account.
Companion tickets are not transferable once issued. Companion Certificate and Delta eCredits can be combined in the same transaction. Delta eCredits can be applied as payment toward cost of the primary ticket, companion ticket, and any government-imposed taxes and fees.
What's the law? The U.S. Department of Transportation (DoT) is clear on the issue of refunds: if your flight is canceled by the airline and you choose not to travel, you are entitled to a refund. That means the dollar amount you paid, not a voucher for the equivalent to use with the airline.
Most often, a voucher or travel credit can be applied as payment toward the cost of a flight. If you choose to cancel a flight, most airlines will give you your money back in the form of a voucher or travel credit equivalent to the value of your ticket.