The defaults.inc file is provided by Silk Test Classic and implements the recovery system for a single application test. That is, it contains the DefaultBaseState function that performs any cleanup needed after an operation under test fails and returns the application to its base state.
You can define a base state function to replace the DefaultBaseState function by defining an application state without using the basedon keyword. This creates an application state that 4Test executes instead of the DefaultBaseState function.
The word "Default" in each of the above function names signifies that you can define alternative functions to replace these. If, for example, you define a function called TestcaseEnter, 4Test will invoke your function before executing any of the code in your test case and will not invoke DefaultTestcaseEnter.
TestPlanEnter() is not called until the first test case in the plan is run. Or the first marked test case, if you are only running marked test cases. Similarly, TestPlanExit() is called at the completion of the last marked test case. TestPlanExit() is only called if the last marked test description contains an executable test case, which means not a manual test case or a commented out test case specifier.