Box1: Private -
In App Center, distribution groups are private by default. Only testers invited via email can access the releases available to this group.
Box 2: Public -
Distribution groups must be public to enable unauthenticated installs from public links.
Box 3: Shared -
Shared distribution groups are private or public distribution groups that are shared across multiple apps in a single organization.
Reference:
https://docs.microsoft.com/en-us/appcenter/distribution/groups