CS is chip select... not used on standard stepper drivers.
From the code
#define X_CS_PIN 53
#define Y_CS_PIN 49
#define Z_CS_PIN 40
#define E0_CS_PIN 42
#define E1_CS_PIN 44
Non of these pins are used on a ramps board. (well they go to aux-2, and aux-3) not used normally.
Looking further threw the code I see
/**
* Enable this for SilentStepStick Trinamic TMC2130 SPI-configurable stepper drivers.
*
* To use TMC2130 drivers in SPI mode, you'll also need the TMC2130 Arduino library
* (https://github.com/makertum/Trinamic_TMC2130).
*
* To use TMC2130 stepper drivers in SPI mode connect your SPI2130 pins to
* the hardware SPI interface on your board and define the required CS pins
* in your `pins_MYBOARD.h` file. (e.g., RAMPS 1.4 uses AUX3 pins `X_CS_PIN 53`, `Y_CS_PIN 49`, etc.).
*/
So these pins are used for SPI-configurable stepper drivers.
From the code
#define X_CS_PIN 53
#define Y_CS_PIN 49
#define Z_CS_PIN 40
#define E0_CS_PIN 42
#define E1_CS_PIN 44
Non of these pins are used on a ramps board. (well they go to aux-2, and aux-3) not used normally.
Looking further threw the code I see
/**
* Enable this for SilentStepStick Trinamic TMC2130 SPI-configurable stepper drivers.
*
* To use TMC2130 drivers in SPI mode, you'll also need the TMC2130 Arduino library
* (https://github.com/makertum/Trinamic_TMC2130).
*
* To use TMC2130 stepper drivers in SPI mode connect your SPI2130 pins to
* the hardware SPI interface on your board and define the required CS pins
* in your `pins_MYBOARD.h` file. (e.g., RAMPS 1.4 uses AUX3 pins `X_CS_PIN 53`, `Y_CS_PIN 49`, etc.).
*/
So these pins are used for SPI-configurable stepper drivers.