All FIN messages conform to a defined block structure. Each block of a message contains data of a particular type and is used for a particular purpose.

Each block of a message begins and ends with a curly bracket, or brace, character '{' and '}' respectively.
All main blocks are numbered, and the block number followed by a colon: is always the first character within any block.

A typical FIN user-to-user message may consist of:

{1: BASIC HEADER BLOCK}
{2: APPLICATION HEADER BLOCK}
{3: USER HEADER BLOCK}
{4: TEXT BLOCK}
{5: TRAILER BLOCK}

• Blocks 1, 2 and 3 relate to header information, Block 4 contains the text of the message, and Block 5 contains trailer information.
• Blocks 3, 4 and 5 may contain sub-blocks, that is blocks within blocks, or fields delimited by field tags, depending on the nature of the message.
• Only Block 1, the Basic Header Block, is mandatory for all messages. Blocks 2-5 are optional and depend upon the nature of the message and on the application in which the message is being sent or received.
• All FIN user-to-user messages contain Blocks 2, 4, and 5.