When design surface, control of TDesignSurface class, is assigned form is placed on it. Form does not activated, when user clicks on the form design surface gets focus. 

Design surface is usual control which can not be placed on any container - form, tab sheet, panel, etc. 

This allows organizing of multiple documents applications where each form is placed on separate tab sheet. 

Also design surface allows hidding of form caption and border by setting TDesignSurface.HideFormBorders property. 

Using design surface you may implement BDS like design environment.

TzCustomFormDesigner Properties