The MTA file resp. its content is subject of the Mp3tag application system and not subject to the common user.
It is not recommended to edit MTA files manually, because of the special sovereign usage by the application itself.
A bad formatted MTA file can cause bad behaviour of the application, which can bring harm to the data and to the user.
If you want to know how to create an action resp. build a group of actions, then read the help manual, and do all changes to the action definitions by using the user interface of Mp3tag.
Yes, you can create an user defined tag-field 'DECADE'.
When the year number is given as format of 'YYYY', what digit is the 'decade digit'?
Or do you want to extract a substring of century and decade digits?
Are you sure to write the text 'Unknown' to the tag-field 'DECADE'?
Be aware of possible formatting problem when displaying the content of the tag-field.
Shouldn't the tag-field better left empty (= not existing)?
DD.20110321.1135.CET