Greg Ondo (Senior Specialist Technical, Steinberg & Yamaha USA) kindly replied to my request. I think it will answer your question regarding the purpose and use of the FX Bus. With regards to the actually implementation, he will show how to set it up in the next Steinberg / Cubase Google Hangout Session, which can be viewed live or thereafter on Youtube (dates will be announced through Club Steinberg Facebook Page), alternatively, just go to the Youtube Channel Club Cubase (https://www.youtube.com/user/ClubCubase/featured) and subscribe.
This is Greg’s replay: "The UR 44 has 2 different Rev X reverbs one that is powered by the onboard DSP of the interface and the other as a native plug-in that is powered by your computer’s processor. Most people monitor the reverb during recording using the onboard DSP as there is imperceptible latency with the effect. People often use the software version afterwards for mixing as it is a great sounding reverb.
The FX Bus allows the already recorded signal to be sent to the hardware DSP to free up computer processing resources which if needed is very convenient. Once activated in the VST Connections you can use an aux send to route the audio to the onboard DSP Rev X."