DFR0327 - is direct communication between Pi and Arduino possible?

I have not been able to find any information about how to get the raspberry pi and the on-board arduino to communicate with each other, except for plugging in a USB cable between the two. For various reasons, I can't have the communication to be done over a USB cable. Is the Pi and on-board arduino connected via something like UART, I2C, SPI, etc.? I guess I could use jumper cables on the existing headers, but I am really hoping that the board comes with the in-built possibility to do it without jumpers or a USB cable.

As shown in the figure, there is a toggle switch at approximately the middle position of the PCB, which can switch whether the UART is connected to the TX and RX interfaces of the Raspberry Pi or the TX and RX interfaces of the Arduino. A diagram indicating the corresponding connections is located at the lower-left corner of the PCB. When the Arduino needs to communicate with the Raspberry Pi, you can switch the switch to position "B". This can also be verified by the toggle switch at the lower-right corner of the circuit schematic.

