An authorization letter is a type of legal document that can allow someone to claim a document on someone else's behalf. Claiming documents is just one potential use of authorization letters, which can also be used to sign documents, make agreements or participate in an event as an agent of another person.
An example of a situation in which someone could draft an authorization letter to claim documents is the person being hospitalized for an extended period of time. He or she may want to designate a spouse or another family member to pick up a passport or other document that is being issued to him or her. In legal terminology, the hospitalized person is known as the "principal," and the designated family member is the "agent."
According to the Law Dictionary, because an authorization letter is a binding legal document, it should be very detailed and specifically outline the actions which are authorized under the letter. It should not be overly broad; for example, designating somebody to handle all of the letter writer's affairs is too ambiguous. It also needs to use the full name of every individual mentioned, set forth the exact dates or a date range during which it is effective and specify any monetary spending limits.