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Launch Test Cockpit (tcode /INT4/IFTT_COCKPIT).
Switch to Change Mode:
Select a testing scenario where new test case should be added.
Add a new line:
Type in Description field the description of the test case.
Select appropriate value for Interface Type field depending on your scenario (you can use input F4 help):
Next step would be to choose an existing Document Number. However, this step differs for different types of interfaces. Therefore, next actions are described in respective interface types sections.