When you make an XLS file act as an add-in -- that was how I built
my first menu-building Excel add-in -- you're supposed to twiddle
the IsAddIn property of the ThisWorkbook object when you edit the
code. That much I remember. But what's the proper sequence for
that. Is it this?

Open the VBE
Change IsAddIn to False
Edit
Change IsAddIn to True
Save (apparently needs explicit, doesn't prompt at close?)

Also, when I forget to do this, it doesn't seem to matter much.
What's the purpose of changing that property? Is it only to
make the add-in not visible? (And if so, why is that necessary
[assuming there's no content on its worksheets] since you can
still get at the code?)

TIA

--
Mark Tangard , Microsoft Word MVP
"Life is nothing if you're not obsessed." --John Waters