When you have a GPU(s) enabled on a system, the last adapter you enumerate will be a 'render only' WARP device. This never has any outputs. You can still render to a window using this device as this doesn't require an output. If you disable your GPU in device manager, or you are running in a VM or HYPER-V, you will (typically) enumerate two adapters, the first is a WARP adapter that will appear to have an output and the second WARP device is the 'render only' device. Note, when WARP is your primary GPU, it also provides D3D9 and below emulation as well.
Yes, WARP is the replacement for Ref. Ref didn't support shared surfaces or kernel synchronization which is being used in a lot of components these days. WARP is *fully* conformant with the D3D specs, so can be relied upon to be a accurate reference. It's also much faster than the original reference rasterizer was, and it's implemented as a 'real device' in the system, so participates in kernel synchronization and supports shared surfaces (which is required for things like the desktop, IE etc...).
Andy