UDs are collection of UIs.
They are used everywhere a group of data is needed: a table's columns, a grid's columns, the fields listed in a combo box, the fields placed inside a form frame, and so on.
If you have a table with several columns, you may want to group this columns into editing groups (UDs), and use those UDs to organize your fields in a form. For instance, you may have all customer fields into a form, Name, Mobile Number, Phone Number, Email, etc...
AND you want all of this fields to exist in a table, so that you can save all this data to a file. In Expert Flux, you simply create a UD for each operation you want and toss all of them inside a single UD - UD Customer Table, in this example. This is the "UD Customer Table" that will be used when you create a table object.