The metadata["type"] codes a notification may carry.

A notification's type is never read by the server — it exists purely so a client can decide how to render and route it. That makes an unregistered code fail silently: the server stores and delivers it happily, and the client simply never handles it. So the set is closed. Gamend.Notifications rejects an unknown code at write time, and this module is the list clients can rely on.

Plugins add their own by exporting notification_types/0 (see Gamend.Hooks.Declarations); those merge with the core codes below.