The category is used to classify the different types of notifications.

pre_shipmentAll the notification sent before a tracker is created. Those notifications are sent using the fulfillment status codes
package_shippedThe package has been shipped by the carrier
failed_delivery_attemptThe package has had an attempt to be delivered, but the delivery did not succeed
other_incidentsThe package has had an incident at some point during delivery
pickup_pointThe package is at the pickup point
package_deliveredThe package is delivered
otherAll other types of notification like the departure to the destination country, out for delivery, at custom etc.
smart_notificationNotifications triggered when a tracker or an order enters a smart state
smart_notification_severeNotifications triggered when a tracker or an order enters a severe smart state