Power cables come with PSUs, not with cases. That is, if you mean the cable from the wall outlet to the PSU.
He's referring to the power on/off signalling cable.
It'll be trial and error I suspect that one is attached the power switch on the case and one to the reset switch, they are both the same type of switches. Once you figure it out cross out the word reset on one of them with perm marker.
Probably a printing mistake, as 13thmonkey said, it's basically the same thing.
Either can go anywhere, and you then will know which one will switch it on/off or reset it. If its the button you don't want, just swap the cables round.
It won't hurt the computer at all.