The default chip select pin (CS) is TX.

TX Jumper

• TX jumper - This jumper is located on the front of the board, towards the center directly above the MicroSD board text and is labeled TX. If cut, the chip select pin (CS) for the SD card is no longer connected to TX and you can solder one of the solder-jumpers closed to change the CS pin.
  
  One of the pins must be used as CS for the microSD card to work

Pin Select Solder-jumpers

• Three solder-jumpers are available to change the CS pin for the microSD BFF. They are located on the front of the board, directly below the microSD card slot. Their corresponding pin names can be found directly below the jumpers. You can choose to solder one of the solder-jumpers for one of the following pins:
  • RX
  • A0
  • A1

CS Pad

• CS - The CS pad is a single square pad, located on the right side of the board between the microSD slot and the Adafruit logo on the silk. You can use this pad to connect the chip select pin to another digital pin.