Explanation
Notification service supports subscriptions topics: E-Mail, Function, Https, PagerDuty and Slack

Alarms: Notifications sends alarm messages when alarms are breached. The alarm message is sent to the topic specified in the alarm. For example, an alarm message might be configured for high CPU usage.
See Managing Alarms.

References:
https://docs.cloud.oracle.com/en-us/iaas/Content/Notification/Concepts/notificationoverview.htm