Can someone explain what these are.

I have a site with a http.ini file in the root of the site. Is the http.ini file always used to define URL rewrites? Also do URL rewrites reply on ISAPI filters being installed on the server?

In the http.ini file there are the following:

RewriteRule /xxxdir_namexxx /xxx.asp\?categoryID=5522 [I,O,L]

Can someone please explain what this mean?

Also, what is [I,O,L]?

thanks a lot,