Well - the LEDs are not statically defined, because you can define their usage by yourself - and the OS can have another default funtion like on a other OS (Orange Pi original Linux <--> armbian)
Your could find the available LEDs here:
root@pihole:~# ls -l /sys/class/leds
total 0
lrwxrwxrwx 1 root root 0 Jan 1 1970 green_led -> ../../devices/platform/leds-gpio/leds/green_led
lrwxrwxrwx 1 root root 0 Jan 1 1970 red_led -> ../../devices/platform/leds-gpio/leds/red_led
and which function is assigned to the LED you could find out while do a "more" an their trigger-file:
root@pihole:~# more /sys/class/leds/red_led/trigger
none mmc0 mmc1 timer [heartbeat] backlight default-on
here you could see I assigned the funtion [heartbeat] to the red LED
I did this in the /etc/rc.local with the following command (before the line with "exit 0") :
echo "heartbeat" > /sys/class/leds/red_led/trigger
heartbeat is this flashing LED - showing the system is running
mmc0 will be like a HDD-LED for your uSD-Card:
echo "mmc0" > /sys/class/leds/green_led/trigger