Somebody who does it on their arms or wrists are what we call "Wemo"'s
a Wannabe EMO. They only do it for the attention, why else would they cut themselves in such an obvious and on show place?
If you're truly self harming not for attention you do it on your thighs because thats alot of skin there that you can easily cut up, which is also not easily seen. Or on their shoulder, because its bony and hurts immensly, as well as not being an obvious place to see the cuts.