Actually thats a good question. I use cartographer and it has it set to yards. Don't know if there is actually a "measurement" used for it. I believe it's can be whatever you want to call it .. can call it smooges if ya wanted
Good thing is is every mod that uses coords are pretty much the same. Always the same coords. I believe it's using the built in loc of WoW. It just displays it for you as it changes.
Good for when people give you coords and places like
www.thottbot.com where coords are always used. Easy way to locate and find stuff