The EDIFACT term for a transaction set. A message is the collection of data, organized in segments, exchanged by trading partners engaged in EDI. Typically, a message is an electronic version of a document associated with a common business transaction, such as a purchase order or shipping notice. A message begins with a message header segment, which identifies the start of the message (e.g., the series of characters representing one purchase order) . The message header segment also carries the message type code, which identifies the business transaction type. EDIFACT's message header segment is called UNH; in ANSI X12 protocol, the message header is called ST. A message ends with a message trailer segment, which signals the end of the message (e.g., the end of one purchase order) . EDIFACT's message trailer is labeled UNT; the ANSI X12 message trailer is referred to as SE.