The document discusses order letters, which are a type of business letter used to purchase merchandise, supplies, and materials for a company. Order letters, also known as purchase orders, provide vendors with details of a company's request, including product descriptions, delivery times, and payment methods. They are an important communication tool between companies and their suppliers. The document also provides guidelines for writing order letters, such as identifying the reader, establishing the objective to make a purchase, determining the necessary scope of information, and organizing, drafting, reviewing, and revising the letter.