It’s under properties in the custom module layout.
I can’t find any documentation for it and it doesn’t output anything in my module
It indicates whether the WYSIWYG has been switched to the CODE editor and had content entered there and saved.
If only the WYSIWYG has ever been used for content editing it will remain false.
Not sure how/why you’d use this in practice, but it’s there 
Just guessing, but I suspect it can be used to prevent the editor opening in WYSIWYG view if switching to WYSIWYG is messing with your code.