There are different options. If you put the power planes on the inner layers and the signal on the outer layers, it's much easier to visually inspect the signa

There are different options. If you put the power planes on the inner layers and the signal on the outer layers, it's much easier to visually inspect the signal layers. And if you got something wrong on the prototype and you have to fix it manually, the traces are right there where you can get at them. On the other hand, if you're doing some performance-critical RF wizardry, you might put the ground planes on the outer layers and the signal on the inner layers, sandwiching your signal between two ground planes. So if you look at a wifi module or something like that, sometimes the outer layers will have very few traces.