Notifications can optionally have a type indicator. Although neither client or nor server must support this, some may choose to. Those servers implementing categories may use them to intelligently display the notification in a certain way, or group notifications of similar types.
   Categories are in
   class.specific form.
   class specifies the generic type of notification, and
   specific specifies the more specific type of
   notification.
  
   If a specific type of notification does not exist for your notification,
   but the generic kind does, a notification of type
   class is acceptable.
  
   Third parties, when defining their own categories, should discuss
   the possibility of standardizing on the hint with other parties, preferably
   in a place such as the
   xdg (http://freedesktop.org/mailman/listinfo/xdg)
   mailing list at
   freedesktop.org (http://freedesktop.org/). If it
   warrants a standard, it will be added to the table above. If no
   consensus is reached, the category should be in the form of
   "x-vendor.class.name."
  
The following table lists standard notifications as defined by this spec. More will be added in time.
| Type | Description | 
|---|---|
| "call" | A generic audio or video call notification that doesn't fit into any other category. | 
| "call.ended" | An audio or video call was ended. | 
| "call.incoming" | A audio or video call is incoming. | 
| "call.unanswered" | An incoming audio or video call was not answered. | 
| "device" | A generic device-related notification that doesn't fit into any other category. | 
| "device.added" | A device, such as a USB device, was added to the system. | 
| "device.error" | A device had some kind of error. | 
| "device.removed" | A device, such as a USB device, was removed from the system. | 
| "email" | A generic e-mail-related notification that doesn't fit into any other category. | 
| "email.arrived" | A new e-mail notification. | 
| "email.bounced" | A notification stating that an e-mail has bounced. | 
| "im" | A generic instant message-related notification that doesn't fit into any other category. | 
| "im.error" | An instant message error notification. | 
| "im.received" | A received instant message notification. | 
| "network" | A generic network notification that doesn't fit into any other category. | 
| "network.connected" | A network connection notification, such as successful sign-on to a
       network service. This should not be confused with device.addedfor new network devices. | 
| "network.disconnected" | A network disconnected notification. This should not be confused with device.removedfor disconnected network devices. | 
| "network.error" | A network-related or connection-related error. | 
| "presence" | A generic presence change notification that doesn't fit into any other category, such as going away or idle. | 
| "presence.offline" | An offline presence change notification. | 
| "presence.online" | An online presence change notification. | 
| "transfer" | A generic file transfer or download notification that doesn't fit into any other category. | 
| "transfer.complete" | A file transfer or download complete notification. | 
| "transfer.error" | A file transfer or download error. |