Version Control System.
To help you to keep track of changes history and allow more than one developer work with the same project Database Designer has a version control system built-in. VCS menu contains the following commands: Check In, Check Out, Undo Checkout, Show History and Show Difference.
To edit object you must check it out. If you have set "Automatically perform Check In/Out operations" flag in your project settings your object will be checked out automatically when you will start editing. Do not forget to check in object when you finish with it.
Show Difference command provides you an ability to compare stored versions of the object to find what changes were made.
Show History dialog shows the list of stored version of the object. You may open selected version for preview and make selected version active: