Azure Active Directory Domain Services (Azure AD DS) now supports two forest types: User and Resource.  In this video, we go over what each Azure AD DS forest type, User and Resource, does and when to use one over the other.

Links

Active Directory, Azure Active Directory and Azure AD Domain Services Explained:
https://www.ciraltos.com/active-directory-domain-service-azure-active-directory-and-azure-active-directory-domain-service-explained/
Deploy Azure AD Domain Service and Join a Server to the Domain:
https://www.ciraltos.com/deploy-azure-ad-domain-service-and-join-a-server-to-the-domain/