Setting this property affects to Root, ContainerWindow and Form properties. 

1. TCustomForm - form designing. 

In this case: 

Root = ContainerWindow = Form = Target. 

2. TWinControl, owned by the form. This control and all it's children controls are edited. Also non-visual components owned by the form may be edited too if AllowComponents is True. 

In this case: 

Root = Form = (Target.Owner as TCustomForm); 

ContainerWindow = (Target as TWinControl); 

3. TWinControl, not owned by the form, for example, TQuickReport. 

This control and all owned by it components are edited. 

In this case temporary form is created as container in design mode. 

Client size of this form is equal to size of the Target. 

Changing form size does not change size of the Target. 

In this case: 

Form = temporary internal Form; 

Root = Target; 

ContainerWindow = (Target as TWinControl); 

4. TCustomFrame. 

Identical to case 3, but changing form size changes size of the frame. 

5. TDataModule. 

Data module and all it's components are edited. 

For editing of data modules temporary Form and temporary container are created, in which designing are performed. 

In this case: 

Root = Target; 

Form = temporary internal Form; 

ContainerWindow = temporary internal Container;

TzFormDesigner Properties