In the press release for Betrayal at Cloud City (75222) it was noted that this set was part of the Star Wars Master Build Series. What is this? Does this replace the Star Wars Ultimate Collector Series?
Both the Master Builder Series and UCS will continue. Here's the official distinction from the LEGO Star Wars team:
The Master Builder Series models are large playsets and beside being complex builds they are characterized by having many play features and functions, interior details as well as a range of minifigures. Ultimate Collectors series will remain highly detailed display models providing complex builds with a focus on authenticity and both Ultimate Collectors Series and Master Builder Series will continue as a way to highlight the unique characteristic of each style of model.
It seems like this distinction should help address some of the confusion from fans that UCS sets such as Assault on Hoth (75098) feel very different than the typical large display models.