From Xojo Documentation
An instance of a ContainerControl can only be embedded into one page.
The ContainerControl is embedded on the passed page and the containing control is the parent of the ContainerControl. The optional Left and Top parameters determine the location of the top-left corner, relative to the containing control, not the parent window. If the ContainerControl itself has Left and Top values (not typical as they default to 0) then they are added to what is specified here. The optional parameters Width and Height determine the size of the ContainerControl.