Azure Active Directory Domain Services (AADDS) fills the gaps of providing domain join, group policy, LDAP, and Kerberos/NTLM authentication to devices that do not natively communicate to AzureAD. AADDS typically includes servers and any devices that rely on LDAP or Kerberos/NTLM authentication.  Additionally, this service provides Active Directory services within the Azure tenant for services like Virtual Machines and even Windows Virtual Desktop.