Consider a scenario, where the client sends a request to add a functionality to Flight Reservation to allow sending an order via email. He also specifies the GUI fields and buttons he wants. Even though, the application is yet to be developed, try and develop a few test cases for this requirement.
A few test cases among the many you could have thought of are listed below
You may have also realized that to create test cases you need to look at something to base your test. This is nothing but Test Basis. This test basis could be the actual Application Under Test (AUT), or may be even by experience but most of the times, like, in this case, would be based on documents.
In fact, this is what happens during the different phases of V- Model.
Where test plans are created using the corresponding documents, and once the code is ready, it is ready for testing.