The Microsoft Display Dock was designed to expand the Microsoft Lumia 950 interface ports while keeping it charged. To achieve this, the Microsoft Display Dock uses USB Power Delivery to communicate with the Microsoft Lumia 950.
Using Granite River Lab's Power Delivery test software we are able to see the communication between the Microsoft Display Dock and the Microsoft Lumia 950.
The Microsoft Display Dock is pretty basic in that it only declares support of 10 watts charging using 5 volts @ 2 amps.
The Microsoft Display Dock also supports DisplayPort Alternative Mode over USB Type-C.
The Microsoft Lumia 950 simply requests 5 volts @ 2 amps from the Microsoft Display Dock.
We can then confirm from the following oscilloscope screenshot that the Microsoft Lumia 950 is pulling 5 volts @ 2 amps from the Microsoft Display Dock while its battery is well depleted.