The Notifier role enables the assignee to send notifications. These include push notifications sent to App users, and email notifications. The users that the Notifier can send notifications to is limited based on the groups and organisational units associated with the role assignment.
You should not rely on push notifications, because there is nothing to stop App users from disabling push notifications for their App via their device settings. As a result, you are recommended to send email notifications in addition to push notifications.