The last parameter is an internalPath - the path within the resource where the content is to be found. It must start with '/' (from javadoc). In other words, it specifies the subpath for the specified resource. For example, you may have a config.jar with follow internal structure:

```
/prod/main.properties
/test/main.properties
```

And with subpath help you can load only one config file:

```groovy
File configJar = new File('config.jar')
StandardRoot root = new StandardRoot()

root.createWebResourceSet(ResourceSetType.CLASSES_JAR, '/WEB-INF/classes', configJar.toURI().toURL(), '/test') // for test env
// or
root.createWebResourceSet(ResourceSetType.CLASSES_JAR, '/WEB-INF/classes', configJar.toURI().toURL(), '/prod') // for production env
```

Of course, if I understand it right :)