Disclaimer:
This information is a believed to be correct, should be as close as it gets;
Also:
Bootloader stuff you should never see, since if it fails kernel doesn't boot. (thus you see no message)

Code Who sets it - Description

0 - any - No error (duh)

1 - bootldr - Unknown exactly, something to do with checking the motherboard

2 - bootldr - Eeprom check failed

3 - bootldr - ??/not used

4 - bootldr - Ram check failed

5 - kernel - HDD not locked (retail bioses require the hd to be locked)

6 - kernel - Cannot unlock HDD

7 - kernel - HDD timeout

8 - kernel - No HDD found

9 - kernel - HDD parameters (PIO/DMA/or size {debug}, certain size minimum is required for debug)

10 - kernel - DVD timeout

11 - kernel - No DVD Founnd

12 - kernel - DVD parameters (PIO/DMA)

13 - kernel - Dashboard launch fail (due to missing/bad key, or anything else that would prevent it from running) and the dashboard didn't specify why it failed.

14 - dashboard - Error loading dashboard (dashboard generic error)

15 - - ??/not used

16 - dashboard - Other files to do with dashboard / dashboard settings (specific dashboard error)

17 - - ??/not used

18 - - ??/not used

19 - - ??/not used

20 - kernel - The dashboard was attempted to load and failed; It was a cold boot, and the dashboard didn't specify why it failed, but it (for some reason) needed to be noted that the dvd passed the challenge/response authentication

21 - anywhere - This error says that the machine was booted to display a error, basically someone told the machine to reboot (or launch a xbe) with this flag, and the error code just means its been rebooted by the flag

In closing:

21 was the highest number I could find, I dont think anynthing is past that. And for the unknown ones, I couldn't find anyreference to them at all, I don't believe they are used.