What are c: and m: in xbindkeys configuration?

In the default example configuration for xbindkeys, there is an example of specifying keycodes directly: >

> # set directly keycode (here control + f with my keyboard)
> #"xterm"
> #  c:41 + m:0x4
>

What do the c: and m: mean? The documentation for xbindkeys does not shed any light on this. Also, I know you can use xbindkeys -k to find the codes for a key. If that does not work (for one of the nonstandard fn keys, the command does not respond to the keypress and it instead does its normal action), does that mean that xbindkeys cannot handle that key? Even if I know the keycode from showkey -k (217 in my case)?