By running tests with iTest and config.tcl, iTest uses built-in (pre-defined) Avalanche API's to start the test using the ports and chassis details defined in the session profile and returns the results, here we can change the parameters dynamically in config.tcl (using the configure option in session window). Whereas by running tests with iTest and TCL test files, iTest executes the test.tcl file (located in the specified directory) using the ports and chassis details defined in the test.tcl file and we can't change the parameters dynamically.