complete List of possible environment keys? (no replies)

Hello,
i understand that most Key=Value pairs in the environment are either
u-boot commands, that are passed directly to uboot, or
script commands, that populate env variables, run other commands etc.

I found a list of uboot commands here, and an explanation on u-boot here. This got me deep into understanding the boot process.

But there are also other things like the LED section
led_error=orange
led_exit=green off
led_init=green blinking
and the "preboot_nc" thing, that leave me clueless.

Also the "device=0:1" line isnt referenced in any other scripts, nor is it understood by u-boot itself.
Which part of the system understands these commands?
Like in case of "preboot_nc", it cannot be the linux kernel, since then the name wouldnt make sense...
Are there more commands?
Is there an (possibly complete) list of all keys , their values and meanings?
where can i dig for those myself?