Sets a relationship to the Style object that will be used to format each of the links to the related topics defined in the related group.
Tip: If you set this to "(auto)", the item will be formatted with the style "Related Items" if that style can be found in the library.
The ItemStyle element corresponds to the "(Styles) For the items use" field, on the Appearance sub-tab, on the Related Topics tab, of the Topic object.