Special keys
Multi_key -- combines multiple characters into one. The "compose" key.
Mode_switch --
scratch pad
setxkbmap -rules xfree86 -model pc104 -layout us -option ""
xkb
rules sepecify small variants.
geometry specifies how keys are physicall arranged, not scancodes. This is only useful for getting pictures of the keys.
keycodes maps scancodes to symbolic names. (<PGDN> = 105;)
symbols go from codes to X symbols (what xmodmap uses). (replace key <RALT> { [ Mode_switch ] };) Symbols associate a physical key with a X symbol. This is basically like setting up the xmodmap table.
References
/usr/X11R6/lib/X11/xkb/rules/xfree86.lst <-- listing of lots of options. There are other lists of options, but I think these are basically duplicating what other venders might have. xfree86 appears to be a superset.