[1] mod_rewrite - Apache HTTP Server Version 2.5 () https://httpd.apache.org/docs/trunk/mod/mod_rewrite.html#RewriteCond

API_VERSION

This is the version of the Apache httpd module API (the internal interface between server and module) in the current httpd build, as defined in include/ap_mmn.h. The module API version corresponds to the version of Apache httpd in use (in the release version of Apache httpd 1.3.14, for instance, it is 19990320:10), but is mainly of interest to module authors.