This is distribution dependent. I have CentOS (a clone of RedHat advanced server) and i have this file.
When you start your machine, the
init process looks at a bunch of scripts to see what to start. One may be
httpd (you can configure apache to start or not with
chkconfig). If you look at your
/etc/init.d/httpd script, you can see that it checks for
/etc/sysconfig/httpd and if so sources it (as if it was a part of the current script). So now any variable definitions in
/etc/sysconfig/httpd get applied for the rest of the script.
The examples you see in the file are to set
HTTPD, which is a variable set to the executable name. In my distro, by default you use the old prefork module, but you can set to use the multithreaded
/usr/sbin/httpd.worker here if you like. You can also set OPTIONS, which are command line options given to
$HTTPD). There really isn't anything else you can set (you can ignore
HTTPD_LANG, if you don't know if you need it, you don't need it)
So, if you want the multithreaded server, set
HTTPD=/usr/sbin/httpd.worker. This probably won't break anything in the default apache, though some add-ons that you add may (but unlikely) break under multithreaded apache.