This is one of the more important directives. It is responsible for mapping module stylesheet MIME types to stylesheet processor modules. (The reason you do this is to make it easy to switch out different modules for the same functionalityfor example, different XSLT processors.)
If you prefix the module name with a + sign, it preloads on server startup ( assuming that the config directive is in a httpd.conf file, rather than a .htaccess file).