So after researching online for some time, here's my understanding of the output's meaning:
Take a look at the KeyPress and KeyRelease events from a letter key, like
KeyRelease event, serial 28, synthetic NO, window 0x6e00001,
root 0x2c9, subw 0x0, time 17599679, (-746,426), root:(337,478),
state 0x10, keycode 58 (keysym 0x6d, m), same_screen YES,
XLookupString gives 1 bytes: (6d) "m"
XFilterEvent returns: False
It has the keycode and reports the window id where the key has been pressed.
Function keys, like Fn+F6, which actually is the brightness decrease key, output no such information. They report 0x0 id for the window and no keykode. Thus, it means these keys cannot be reassigned and are controlled by software , like the X server or console; they are dependent on the environment in which they run. Which also somewhat explains why the Super key may or may not work within a particular desktop environment - it's up to the developers to include functionality of that key into their desktop environment.
MappingNotify event, serial 28, synthetic NO, window 0x0,
request MappingKeyboard, first_keycode 8, count 248
KeymapNotify event, serial 29, synthetic NO, window 0x0,
keys: 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0