“Respond to all (known and unknown) client computers” setting behaviour
If you select “Respond to all (known and unknown) client computers”, and check the box “For unknown clients, notify administrator and respond after approval”, the result is:
When PXE booting a client, it will wait for you to approve it. Click Pending Devices in the WDS console, right-click the pending devices in the right pane and click Approve. The client computer continues with the network boot operation and a computer account object is created in AD DS to represent the physical device. Press F12 on the client and boot into a boot image. Before loading install images, you will be prompted to enter a domain user account name and password. This domain user account does not need to belong to a domain administrator groups.