Let’s say that you’re running a published encounter (like one of the ones in the mini-adventure in the back of the DMG) but when you’re PCs arrive at the encounter, they don’t quite match the specs that the encounter was designed for. Their level might be different, or there might be more or fewer PCs than the encounter was designed for. You could run the encounter as is, but that would lead to some sort of hiccup in the character advancement, or would make the encounter harder or easier than the module planed on. So, to get rid of that problem, you decide to scale the encounter, a process that’s supposed to be really easy in 4e. And when it comes to levels, it is. Party level too high? Bump every monster in the encounter up the necessary number of levels. Too low? Bump them down. Every monster is supposed to be easily manipulatable through an approximately 10 level range (5 up, 5 down) and there are guides for that in the DMG.
But what about numbers? Say you have a large group of 8 PCs. Or maybe some of your players didn’t show up and you’re down to three PCs for the evening. How do you go about scaling those stock encounters that were designed for 5 PCs?