Seeds Studio Round Display & CircuitPython : using SD card + TFT

Thanks, nice question as I realise I do not understand yet this step of the code:

TFT case, I use:
spi_bus = busio.SPI(clock= board.D8, MISO=board.D9)
displayio.FourWire(spi_bus, command=board.D3, chip_select=board.D1)

SD card case, I use:
spi_bus = busio.SPI(clock= board.D8, MISO=board.D9, MOSI=board.D10)
cs = digitalio.DigitalInOut(board.D2)
sdcard = adafruit_sdcard.SDCard(spi_bus, cs)