In the Q3 source it is used twice - once on level load and in G_Spawn where it is called whenever an entity is created.

• keynum : A key keynum.

The values of keynum range from 0 to 255, inclusive.
For the most part, a key's keynum corresponds to its character's ASCII value (e.g. the 'A' key is 65).
I have a list of keynames used by the bind command with their keynums which I may append at a later date.
In the meantime, if you want to know them now, you can do this:

Create a keynums.cfg with a pattern like:
bind 0x00 Key 0x00
bind 0x01 Key 0x01
...
bind 0xfe Key 0xfe
bind 0xff Key 0xff

(case matters for the hex values, and a scripting language to automate this really helps)

Back up your q3config.cfg, run Q3, "/exec keynums.cfg; bindlist; condump keynums.txt; quit"

In the file "keynums.txt" you should now have lines like:
UPARROW "Key 0x84"

There's your keynum list.

Returns qtrue if the indicated key is held down, qfalse if not held down (released).

PhaethonH