This event is signaled by act_subscription() in inc/actions.php when receiving a subscribe request, handlers can use it to allow different subscription styles, targets and actions.
The passed Doku_Event object has the fields: $data and $result.
The $data field is an array with the entries:
:.subscribe and unsubscribe.
In the BEFORE event handler you can modify the $data field and eventually you can use here preventDefault() to skip execution of DokuWiki's default subscription_handle_post().
The $result field is always null because this event has no default action that sets this field. The AFTER event handler let you process the $data field further after the default processing.