Backlight responds to layers

keymap.c

bool has_layer_changed = true;

void matrix_scan_user(void) {
  uint8_t layer = biton32(layer_state);
  static uint8_t old_layer = 0;

  if (old_layer != layer) {
    has_layer_changed = true;
    old_layer = layer;
  }

  if (has_layer_changed) {
    has_layer_changed = false;

    switch (layer) {
      case _LOWER:
        backlight_set(1);
      break;
      case _QWERTY:
        backlight_set(2);
      break;
      case _RAISE:
        backlight_set(3);
      break;
      case _ADJUST:
        backlight_set(4);
      break;
    }
  }
};

where does one specify what corresponds to the backlight_set 1..4?
i've got working it working for underglow with rgb layers, where the number is a array index to the specified layer color configuration. but can't seem to find any information for the backlight scenario.

Example:

const rgblight_segment_t PROGMEM mg_func_layer[] = RGBLIGHT_LAYER_SEGMENTS(
    {MG_LAYER_LEDS_RIGHT - 2, MG_LAYER_LEDS, HSV_GREEN}
);
...
const rgblight_segment_t* const PROGMEM mg_rgb_layers[] = RGBLIGHT_LAYERS_LIST(
    mg_capslock_active_layer,
    mg_func_layer,
    mg_caps_layer,
    mg_menu_layer
);

...
layer_state_t layer_state_set_user(layer_state_t state) {
    rgblight_set_layer_state(1, layer_state_cmp(state, _FUNC));
    return state;
}

so the 1 parameter to rgblight_set_layer_statecorresponds to themg_func_layer rgblight_segment_t`.
trying to understand the same relationship here :)

I believe this is ancient code. QMK's lighting support has gotten a lot better.

https://beta.docs.qmk.fm/using-qmk/hardware-features/lighting/feature_rgblight