This defines the "look" and to some extent content of the output for Delphi source code. If 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. Typically, you don't need to set this value.
The property resides in the Project tree view under the Outputs node, on the output nodes where it applies.
Applies to: WinHelp,PlainHTML,HTMLHelp