Right well if you are talking about customs like KMAC, LZ, etc... those are programmable so you can do whatever you want. For stock boards like vintage G80-1000 or something, you can use some kind of remapping utility like sharpkeys but you have to decide what other key to sacrifice on the reassignment since you can't physically add any extra keys to that. For example when I am using old winkeyless Cherry board I may reassign caps lock to Windows since I find Windows key very indispensable on Windows 8.1 while the caps lock key is essentially useless to me. If I want to get context menu I just use the mouse and right click.