Explanation
Alert policies let you categorize the alerts that are triggered by a policy, apply the policy to all users in your organization, set a threshold level for when an alert is triggered, and decide whether to receive email notifications when alerts are triggered.
Default alert policies include:
Unusual external user file activity - Generates an alert when an unusually large number of activities are performed on files in SharePoint or OneDrive by users outside of your organization. This includes activities such as accessing files, downloading files, and deleting files. This policy has a High severity setting.
Reference: https://docs.microsoft.com/en-us/microsoft-365/compliance/alert-policies