[1] mod_rewrite - Apache HTTP Server Version 2.5
()
https://httpd.apache.org/docs/trunk/mod/mod_rewrite.html#RewriteCond
IS_SUBREQ
Will contain the text "true" if the request currently being processed is a sub-request, "false" otherwise. Sub-requests may be generated by modules that need to resolve additional files or URIs in order to complete their tasks.