The second authentication option, ADFS, is the default option for disconnected scenarios. ADFS interoperates with your existing Active Directory, and therefore, authentication with existing accounts is possible.

In the set of virtual machines running within your Azure Stack setup, a dedicated ADFS VM will be needed. If you are using ADFS within your Azure Stack implementation, you are bound to one ADFS trust. This means there is no chance of connecting your customer's Active Directory to your cloud service using ADFS directly. If you need it, you will have to set up an ADFS trust to a domain service in between, which then directly connects to your Azure Stack environment using another ADFS hop.