A proxy directive simply identifies the individual that is trusted to make surrogate decisions if the person is not capable of making decisions. A heath care proxy and a durable power of attorney for health care are examples of proxy directives.
Depending on the jurisdiction, local laws may grant specific legal status to either kind of directive. When it does so, it usually requires a specific format and specific procedural rules. Even when there is no law however, such directives have moral force as representation of the person's wishes in regard to future health care.
As a practical matter, it is often difficult for people to anticipate the specific medical decisions that may arise in the future. A proxy directive will still permit decision making by a trusted agent, but it is important that the surrogate at least be familiar with the wishes, values and attitudes of the patient. Frank discussions between patient and agent over time are helpful. An instrument which further facilitates this is a values history.