The dialog box opens when you click the Insert Macro button in the Edit Tool dialog box.

ItemDescription
MacroThis section contains the list of available macros to be used to define the external tool's program, parameters, and working directory.
PreviewThis section shows the preview of the selected macro, in the context of the current project/module.

In case of multiple projects, opened in the same window, the macro ModuleSdkPath expands into an SDK that corresponds to the current project.