This event is signaled by generate() in inc/Sitemapper.php when a new sitemap is generated, handlers can use it to change the generated XML, prevent saving the sitemap and provide their own routine for saving the file and add or remove items to/from the sitemap. The path the sitemap will be saved to can be changed, but shouldn't be changed as the path getFilePath() returns is used to deliver the sitemap.
The following plugins are known to handle this event and their source code may be a good start for understanding and implementing a handler yourself.
