Trailers are added to a message for the following reasons:

• for control purposes,
• to indicate that special circumstances apply to the handling of the message,
• to convey special/additional information.

They may be added by the user or by the FIN service.

For example, Block 5 of a user-to-user message, sent with a MAC and a Checksum Trailer, may appear as:

{5:{MAC:41720873}{CHK:123456789ABC}}

Some other possible fields are:

• MAC Message Authentication Code calculated based on the entire contents of the message using a key that has been exchanged with the destination and a secret algorithm.
• CHK Checksum calculated for all message types.
• PDE Possible Duplicate Emission added if user thinks the same message was sent previously.
• DLM Added by SWIFT if an urgent message (U) has not been delivered within 15 minutes, or a normal message (N) within 100 minutes.