UD - Unity of Data
An UD is a collection of UI´s.
An UD is like an Excel table and the expanded UIs inside it are the columns.
An UD is used whenever a group of UIs is needed. To create that group, you create a new UD, clicking on the UD symbol in the toolbox. After that you can drag UIs and other UDs from the workspace into the new UD´s lane (The lane is the soft colored area at the right of the new created UD).
See also Expanded UIs