Services are more difficult to test because they often depend on the data status and not all eventualities can be predicted with certainty here. In addition, since services use the same functionalities that are already covered by unit tests, duplications potentially arise here. An example would be that numbers are formatted as degrees Celsius, which is a base function. Since the service tests all potentially contain one or more measurement specifications, one test for each case would not create any additional security here, but would have to revise not only the base test but also all service tests if the representation were adapted.
In the third level, which primarily represents the presentation of information and the input mask (GUI), it makes sense to reduce the tests. In contrast to the strongly linear processes of the basic functions, user interfaces are much more complex, since inputs can be entered independently with much more freedom. However, tests at this level can only run defined processes and should therefore only be used for the most important and known (or assumed) processes.