I'm not aware of a central source, but this is what we were told several years ago by a great moderator that looked into it:

• Elections that are built-in to the program (it has a form for it) are e-filed.
• If there is a button that says something like "explain" or "statement" it is e-filed (for example, line 53 of Form 5329 or on Form 1116).
• The "preparer notes" are e-filed.
• That is pretty much everything.  The "notes" are NOT e-filed.