I have a new Brother MFC and had permission issues and managed to solve them by changing GROUP=scanner to GROUP=lp in the relevant udev rule as my user is in the lp group but not in the scanner group and I didn't want to log out and in. However /dev/usb/lp0 was lp before and it still is. So it must be something else, but what? 
What does GROUP in an udev rule actually set?
udev
You have probably changed the rule pertaining to the /dev/bus/usb/<busnum>/<devnum> device nodes. Those are generic USB device nodes, which can be used from user-space programs to communicate with any USB device (even those that do not have any specific driver).