RGBLIGHT_SPLIT: This option enables synchronization of the RGB Light modes between the controllers of the split keyboard. This is for keyboards that have RGB LEDs that are directly wired to the controller (that is, they are not using the "extra data" option on the TRRS cable).
RGBLED_SPLIT: This sets how many LEDs are directly connected to each controller. The first number is the left side, and the second number is the right side.
These are the three main initialization functions, listed in the order that they're called.
keyboard_pre_init_* - Happens before most anything is started. Good for hardware setup that you want running very early.
matrix_init_* - Happens midway through the firmware's startup process. Hardware is initialized, but features may not be yet.
keyboard_post_init_* - Happens at the end of the firmware's startup process. This is where you'd want to put "customization" code, for the most part.
Note: For most people, the keyboard_post_init_user function is what you want to call. For instance, this is where you want to set up things for RGB Underglow.