Datasheet

Starting from the left you've got

SPI Clock (SCK) - output from feather to wing

SPI Master Out Slave In (MOSI) - output from feather to wing

SPI Master In Slave Out (MISO) - input from wing to feather

These pins are in the same location on every Feather. They are used for communicating with the SD card. When the

SD card is not inserted, these pins are completely free. MISO is tri-stated whenever the SD CS pin is pulled high

The SDCS pin is the chip select line.

On ESP8266, the SD CS pin is on GPIO 15

On Atmel M0 or 32u4 it's on GPIO 10

On Teensy 3.x it's on GPIO 10

You can cut the trace to the default pin and change this to any pin by soldering a wire to any available pad.