
Explanation:

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