LEDE LED configuration on Omega2+

I want to create a custom hardware using the Omega2+ board. On the Expansion-Dock schematic, there is an RGB LED connected to GPIO lines 15,16 and 17. However, looking at the dts files the system LED is configured to GPIO 12.

	system_led: system {
		gpios = <&gpio1 12 GPIO_ACTIVE_LOW>;

Moreover, this is labeled as an amber LED, that suggests that the RED and BLUE colors are used together.

Can you give me a hint what is going on here? Why the configuration doesn't match? When designing the new board, where to put a single LED?

And... when I build LEDE for other hardware (WIZFI630A) there is no such thing "system LED". What we have is run LED. How does the system know which LED to use?

Thank you for your help.