A DB design file corresponds to a "table" in a typical database.
To create a new DB design file (extension: .ufs), click on the menu icon 
(+ button). file, it will be added to the DB definition list.
If necessary, change the DB name in the [Object] pane of the Properties Area => [Basic Information] tab => [Data Name].
If necessary, you can use the menu icon
(Save) to save the data and item information, as well as their associated settings, to the DB design file (the same can be done by selecting [File] ⇒ [Save Selected File] from the TOP menu).
Also, 
(Save All) will save all modified files, including the DB design file (the same applies if you select [File] ⇒ [Save All] from the TOP menu).