The FSGEmail Application is self-testable as is the Hotel Application, which you will see later, is built-on the FSGEmail Application. The Email case is strictly meant to be used as a spun-off subcase. For the FSGEmail Application to test itself, the Email case must spin-off another Email case as a subcase. The Email case does not spin off a subcase when HasCover = true.
The data that the parent Email case propagates to a test-mode-only Email subcase is the same data that ANY parent case would propagate. The data propagated to the reusable Email case constitutes its "API".