The User Header is an optional header available within FIN for user-to-user messages only, and allows users to provide their own reference within the headers for a particular message.

A User Header can only be assigned by the sender of a message and, if assigned, will always appear on the output message copy. Relevant parts of the User Header will be repeated in related system messages and acknowledgements.

Block 3 may contain different combination of tags, so it is not a fixed length string as seen for blocks 1 and 2.

The following it's and example of User Header in a FIN message:

(a) Block Identifier
The Block Identifier for a User Header Block always has the value '3:'.

(b) Banking Priority
Tag 113 defines an optional 4-character banking priority.
The contents of tag 113 are agreed between two or more users.

(c) Message User Reference (MUR)
Tag 108 defines a free-format field in which users may specify their own reference of up to 16 characters of the permitted character set.