This event is signaled by pageTemplate() in inc/common.php when a new page is to be created (loaded into the editor), handlers can use it to load their own namespace templates or manipulate loaded data before being used by the editor.
Plugins usually want to intercept the BEFORE event. If your plugin sets either the tpl or the tplfile info, the usual template lookup will be skipped and your provided data will be used. Setting doreplace to false will skip the pattern replacements.
Intercepting the AFTER event will give you access to the template data after replacements have been done.
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.
