Messages are represented by a three-digit number, for example MT 103, where the first digit defines the Message Category, indicating the general usage of the message.

There are nine categories of user-to-user message, identified as categories 1-9, and a separate category for messages exchanged between a user and the service, identified as category 0.

• System Messages. MT category 0, which may relate to the sending and receiving of messages, for example, user-to-FIN system messages, Delivery Notifications or Retrievals, or to some aspect of a user's LT or destination, for example, LT History Requests, Redefinition of Delivery Subsets.
• User-to-User Messages. MT categories 1-9, which enable users to perform financial transactions.
• Service Messages or Control Messages, which relate to system commands, for example, LOGINs, or to acknowledgements, for example, ACKs, SNKs, or positive user acknowledgements (UAKs)..