A typical use for custom page style templates is to add project specific header and footer to pages, say a company logo, project name/logo etc. If this property is left empty, the default output template (relative to this output engine) will be used. You are not expected to include the ".tpl" extension in the name.
The property resides in the Project tree view under the Outputs node, on the output nodes where it applies.
Applies to: PlainHTML,HTMLHelp