I don't see any mention of this feature in the documentation.
Is there some way to include a label, in plain language, so that the function of the filter expression can be easily identified in the dropdown menu?
Complicated filter expressions are sometimes hard to "read", especially if they were created long ago and/or are seldom used.
Jim
+1
Think this feature should be a future addition to the program.
No, currently not.
But, as a workaround you could create at a place of convenience in your file system a plain text file into which you copy the filter expression and add an explaining text.
In case you need a seldomly used filter expression you could consult your text file and copy&paste it back to the filter entry box in mp3tag.