By default, the remote debugger negotiates the strongest Windows authentication provider that can be used between the Visual ...

By default, the remote debugger negotiates the strongest Windows authentication
                provider that can be used between the Visual Studio computer and the remote debugger's
                computer. Today, this will generally be Kerberos authentication if both computers
                are on the same domain. This is done to provide the best possible security.
                
The '/ntlm' option instructions the remote debugger to instead use NTLM authentication provider. This option is generally not recommended. However, it can be very useful in situations where Kerberos is failing to authenticate.