in reply to
The behavior is [sic] undefined
Well, look, all of these phrases are just euphemisms for what's really going on in the head of the language designer:
This feature should not be considered a feature because I am too stupid or lazy to figure out how it really ought to work, and I reserve the right to change my mind about it in the future, at which point everyone will be required to believe that the mess is all your fault, not mine, for charging in where the language designer fears to tread.
But that's kind of unwieldy and tiresome to type, given how often this language designer is stupid, so whenever you see one of the shorter versions, just substitute the paragraph above. Thank you. :-)