An approval letter can vary greatly in its purpose, but, in general, an approval letter should have a formal, relaxed tone, a clear confirmation of approval, relevant reasons behind the approval, any further actions required of the recipient and important dates or timelines. A letter of approval can also be congratulatory, depending on the appropriateness.
Approval letters can be used in many situations. For instance, an employer may write an approval letter to an employee who requested a vacation. Or, a administrator may write a letter of approval to a teacher requesting permission to take his or her students out on a class trip.
In any case, an approval letter should have a clear focus, making sure the recipient understands that he or she has been granted an approval to the particular request. Also, it's very important for an approval letter to offer important details such as dates, stipulations and other relevant information, to the recipient. It's also important that an approval letter has proper grammar, spelling and sentence construction, so the information in the letter is effectively communicated to the recipient.
Approval letter oftentimes must be professional, in nature. Subsequently, the tone of an approval letter may have to be formal. However, a formal letter can still have a warm, relaxed tone. Approval letters may be congratulatory, too, if the situation is applicable.