mod_logio.c

Just took the plunge and upgraded to 2.0.46.
However, in the httpd.conf are these 2 lines,

# You need to enable mod_logio.c to use %I and %O
#LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\" %I %O"
combinedio

My first question is, what are %I & %O for?
Second is, the top line says mod_logio.c, but in my modules folder I only have
mod_logio.so (is "c" for Linux and "so" for windows?)
And thirdly, as there is no entry in the list of modules for to be able to turn
this module on, what are the necessary parameters after LoadModule?

TIA
Brian