CR "Fuck You"
CR "Fuck You" (Challenge Rating "Fuck You") is the Challenge Rating reserved for creatures who are way too strong for you to defeat except under extraordinary circumstances.
These creatures, sometimes called CRFU creatures, oftentimes don't even have stats. If you try fighting them (again, except under extraordinary circumstances), you probably won't even roll initiative. Or, if you do, they'd almost certainly go first and clobber you in a single turn.
Most CRFU creatures just straight-up do not have actual Challenge Ratings. Like, it will literally just say in the source book, "Yeah, the players can't kill this guy. Tell them they shouldn't even bother trying." There won't even be a number to even tell the players.
Some CRFU creatures, however, do actually have Challenge Ratings (as in, an actual number!), but these are oftentimes so high that knowing it won't actually make you (i.e., players) feel any better.
Now, all that being said, there are some CRFU creatures who, despite having no stats, might have an aspect, an avatar, a projection, or some other lesser form of themselves that does have a CR that is not absolutely ridiculous, allowing players to challenge such entities in an indirect manner.
Examples of CRFU creatures include:
