SWIFT MT field format rules

Can someone point me to a document where the formatting rules for a MT message field are described?

For example I want to know what the following snippet means.

:4!c//3!a/3!a/15d

This is described in the MT 'General Information' documentation. You can find the 2017 edition of this document here. Specifically, check out section 6.1 for an explanation.

How to read :4!c//3!a/3!a/15d:

• first :
• then 4 mandatory c
• then //
• then 3 mandatory a
• then /
• then 3 mandatory a
• then /
• then maximum 15 d

A brief overview of the character types in regex:

• c is [A-Z0-9]
• a is [A-Z]
• d is \d+,\d*