Unify multiple HID interfaces into a single Endpoint.
This reduces the number of USB endpoints required, which frees them up for other things.
NKRO and EXTRAKEY always use the shared endpoint.
By default, MOUSEKEY also uses it. This means it won’t work as a Boot Procotol mouse in some BIOSes, etc. If you really think your keyboard needs to work as a mouse in your BIOS, set MOUSE_SHARED_EP = no in your rules.mk.
By default, the core keyboard does not use the shared endpoint, as not all BIOSes are standards compliant and that’s one place you don’t want to find out your keyboard doesn’t work.. If you are really confident, you can set KEYBOARD_SHARED_EP = yes to use the shared endpoint here too.