The digital outputs that are available on some of the variants of our displays are open-drain outputs.
This means that they are not actively pushing any voltage.
Instead, they connect an external power source to ground, closing the circuit and thus enabling the external element.
The external VCC and the power source of the display device can be the same.
The grounds have to be connected (i.e. have to be the same).
The LED in the image is only an example, it is not limited to LEDs.