What renders currency "unfit to be reissued?"?

"Under Title 18, Section 333 of the United States Code....currency defacement is generally defined as follows: Whoever mutilates, cuts, disfigures, perforates, unites or cements together, or does any other thing to any bank bill, draft, note, or other evidence of debt issued by any national banking association, Federal Reserve Bank, or Federal Reserve System, with intent to render such item(s) unfit to be reissued, shall be fined under this title or imprisoned not more than six months, or both."

So, my question: what does "with intent to render it unfit to be reissued" mean exactly? Does that mean you can do anything whatsoever to a bill as long as you are still able to discern that it's US currency as long as you had no "intent" to make it unfit? Could you paint over everything on it except the denomination, serial number, etc, and still be able to use it? I don't get what "unfit to be reissued" actually means and in my searches, on the US Mint page and the US Secret Service Page, and even on the wheresgeorge.com page (including here, where the owner answered someone's similar question) they just repeat the same phrase. There seems to be no definition of the actual meaning. Does it mean you can do anything whatsoever to the bill as long as somehow it ends up being recognizable as official currency? So, can you shred it into jigsaw puzzle pieces, color them all differently and draw and write all over it, and put them back together, just keeping enough showing that you see it's a bill, see the serial number, series, denomination, etc, and that's okay? I see no definitive answer on the Internet. Has anyone found one?