Do you mean UI as in X forwarding? I thought this wasn't possible, which kept me from using mosh so far.
They mean UI as in for command lines, like vim/emacs/shells
TUI, better?
UI is just plain ol' things for user to interact with computer.
The "G" in "GUI" stands for "graphical". Some people use "TUI" to mean "Terminal User Interface". It therefore follows that someone using merely "UI" has not specified the type of interface.