The values of the ISEN rating run from 1 (mildest) to 7 (most severe). A value of 0 is used for a negligible event with no safety impact — no one was reported to be killed, injured, or exposed, no radiation was released, and no equipment was damaged. My "fake" value of 8 would presumably result from a meltdown of all the reactors in the world, threatening the entire population. That's fake, all right.
The responsible nation in a weapons incident could in principal be any of the "Big Five" — the USA, the USSR/Russia, the UK/England, the PRC/China, and France — or, more recently, Israel, Pakistan, India, or North Korea. In practice, only the first three have weapons incidents in this list. When it comes to reactor or radiological accidents, however, the "Big Five" are joined by many other nations.
This power of ten denotes the nearest population range which includes the reported number of people affected. For example: "1" means 10 or fewer affected; "2" means 11 to 100 affected; and so on. The "fake" value shown ("10") indicates a range of ten billion people. This includes everyone there is. Which is why it's absurd. But if you ever do see it, you'd better fill your pockets with jam, because you're about to be toast.