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.

• $data['target']: Target page or namespace ID; Namespaces are denoted by a trailing ':'.
• $data['style']: Subscription style; core allows 'every' and 'digest' for pages and namespaces, and 'list' for namespaces.
• $data['action']: Subscription action; core knows 'subscribe' and 'unsubscribe'.

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.

• none

• Action Plugins
• Events