Understanding the MKS Gen L V1.0 and GRBL

Hi, I’m posting this here because all other avenues have yielded no help so please excuse me if I’ve overstepped.
I have identified the above board as something I would like to explore some projects in using GRBL.
I want to connect a diode laser module to the board, and these usually have 3 output pins, 12v +/- and PWM.
Accordingly the seemingly best available PWM output on the board, although I stand correcting, is P2.5, a large mosfet.
In the documentation for smoothieware it shows how to allocate the pin.
However, in GRBL I am presented with the following:
// The LPC17xx has 6 PWM channels. Each channel has 2 pins. It can drive both pins simultaneously to the same value.
//
// PWM Channel PWM1_CH1 PWM1_CH2 PWM1_CH3 PWM1_CH4 PWM1_CH5 PWM1_CH6
// Primary pin P1.18 P1.20 P1.21 P1.23 P1.24 P1.26
// Secondary pin P2.0 P2.1 P2.2 P2.3 P2.4 P2.5
#define SPINDLE_PWM_CHANNEL PWM1_CH6 // BED MOSFET (P2.5)
#define SPINDLE_PWM_USE_PRIMARY_PIN false
#define SPINDLE_PWM_USE_SECONDARY_PIN true
#ifdef SPINDLE_PWM_PIN_1_23
#define SPINDLE_PWM_CHANNEL PWM1_CH4 // MOSFET3 (P1.23)
#define SPINDLE_PWM_USE_PRIMARY_PIN true
#define SPINDLE_PWM_USE_SECONDARY_PIN false
#endif
#ifdef SPINDLE_PWM_PIN_2_4
#define SPINDLE_PWM_CHANNEL PWM1_CH5 // MOSFET3 (P1.23)
#define SPINDLE_PWM_USE_PRIMARY_PIN false
#define SPINDLE_PWM_USE_SECONDARY_PIN true
#endif

I haver no idea where to go with this as I am new to these languages, although it is of definite interest as I would also like to run a machine spindle from the board through the same machine framework.

Could someone explain to me the necessary steps I need to take please to make this work?

Apologies for the length of the post, Laurence.

Okay so I’ve made some progress on my own but could someone explain the above statement in GRBL to me please?
Many thanks, Laurence.