When a user fills out the Content Notify form, they are essentially creating a subscription. When something on the website is published/updated that matches a user’s subscription, an alert is sent to them in the form of an email.
Alerts can be triggered by two things at this time: when content is published, or when content is updated. This can be done using the Block Editor or Gutenberg plugin, the Classic Editor plugin, via the WP REST API, or via WP-CLI.
Content that is set to private will not trigger a notification. Content that is set to public and is also password protected will trigger a notification but will not include the post content in the message body of the alert.