FACE Base is a base of FACES Kit. It's composed of M5Core base and battery as shown below.
There are some expanding gpio(see the last picture) of M-Bus at the back of FACES Base, in order for you make a complex project conveniently. Obsolutely, a battery(600mAh) is built in base, so that you can use FACES Kit without usb cable.
Please stack M5Core and FACES Keyboard(i.e. QWERTY, GameBoy, Calculator) over Base when use it as shown below**