Thank you for your answer Paul.
My suggestion for addition is for this page in aTbRef, which deals specifically with Exploding Notes:
Under the topic “Delimiters you can use include:” it says (also)
“Otherwise use a custom string of your choice, e.g. a sequence of characters unlikely to occur in normal copy, e.g. ‘####’ or ‘XYXYXY’, etc. The text is case-sensitive. Use of regular expressions is not supported for this option.”
I wasn’t (and still am not completely) aware what “regular expressions” are. To me, according to this text, *** would be a possible “custom string of your choice”. It’s not that much different from #### , at least not to me (and maybe other users).
I think it would be useful to list (also here) all the characters (or regular expression special characters) that should not / cannot be used to explode notes.
Or maybe even add some code to the program to interpret these input errors and give a detailed warning to the user.