Sets a relationship to the Style object that will be used to format the Related Group Heading in the published output.
Tip: If you set this to "(auto)", the Related Group Heading will be set to the style "Related Heading" if that style can be found in the library.
The HeadingStyle element corresponds to the "For the group heading use" field, on the Appearance sub-tab, on the Related Topics tab, of the Topic object.