Is there a way to modify an existing environment variable in MP3Tag?
You could call an external CMD-Batch to achieve this.
But I'm not sure if such a change would survive a Windows reboot.
Why would you like to change an environment variable with Mp3tag?
I want to remember the track where I stopped the processing.
You could write a simple export file with the filename or complete path name?
Or you could set a user-defined tag field that is only present in processed files.
A filter in MP3tag would then show all the files without that marker.
There are sooo many ways to keep track of processed files, depending on your work flow.
My method relies on three folders and on applying a unique file name style at the end. The folders used are: Current Downloads, M4aRaw, and M4aDone.
First I move ten files from the Current Downloads folder to M4aRaw for tagging. After all ten files are tagged or edited, I convert the tags to file names with this simple format:
%artist% (%year%) %title% (%vocalist%)
The resulting names are always very different than the raw input file names, so there is no possibility of confusion. I then move the fully processed files into the M4aDone folder. Then the cycle begins again for the next ten files from the Current Downloads folder.
So by glancing at the three folders, I can see exactly what I have done and what remains to be done for a given project. No special variable is needed.
Dear fellow travellers,
Thank you very much for your creative ideas.
The information is also needed at operating system level to continue dependent processes there. Therefore, these ideas are not the solution for the task mentioned.
You can always use the "Date Modified" to sort and get anything last changed looking outside of mp3tag.
Yes, Motley you're right, the "Date modified" can be helpful.
But in the given case it's not that easy as the other processses don't touch the date stamp,they work on a directory level. 
I loved to hear from your experience but my search isn't finished yet.
But this has no concern for mp3tag as only files can be actually updated with new detail and that can be marked by the modification date. Yes the directories can be changed as well, but this would be a byproduct of the file changes.
You are very vague in respect to what the requirements of the
may be. I find it particularly strange that these processes rely on system variables instead of easily accessible attributes like updated information from the filesystem.
A system variable would be something managed by the OS - so it would require an OS command to set it.
The most straightforward way would be to
You have not yet told us why that method does not work.
I have no idea how to modify an CMD-Batch from inside MP3Tag.
How can this be done?
You use a Batch CMD file to set your global variable with
SET MyGlobaVariable="Whatever you want so save"
Call the external Batch CMD file containing above line in an Option -> Tools entry.
Replace the fix text with a variable from Mp3tag and pass the content of this variable to your Batch CMD.
As long as you don't tell us exactly what you want to store in such a Global Variable we can only guess.
And BTW:
How do you process this Global Variable once it has been set?
I have just understood that the setting of an environment variable is a manual function call and cannot be done via a predefined action.
What is the consequence?
If the system variable is an absolute must then you have reached a dead end right now (so apparently, there is no solution).
What will your workaround look like?
Or isn't the system variable mandatory any more?
I don't think that understanding that no Mp3tag action can directly set an environment variable (for your OS) is a solution to your original question:
We offered several solutions, including how to modify an existing environment variable and other "remember the track where I stopped processing" workarounds.
But none of them seem to fit your (still unclear) request, and we still don't know why they don't fit your needs.
I understand that the answer to my original question is:
NO, an environment variable cannot be changed via automatic procedure from inside MP3Tag.
That made me realise that this approach was no longer effective for me.
This answer is not true.
It is true, that Mp3tag can not set an environment variable directly with an Action.
But you can use a Tool like this:
After calling this Tool you will find the environment variable
LastProcessedFileName
in your list of environment variables like this:
The used command SETX is available since Windows XP:
Can this be done inside a procedure? 
There is no such thing as "procedure" in Mp3tag.
Can this be done with an Action: No.
Can this be done with a Tool: Yes.
Can a Tool have a keyboard shortcut like Ctrl + 1: Yes.
There is no way to guess when you want to finish your work in Mp3tag and write the last processed filename. We don't have a procedure like "on Mp3tag exit do this or that".