When you write an application that runs TestStand sequence files, you do not handle the details of executing each step. Instead, you call the TestStand Engine to begin executing the sequence file in a new thread. The engine notifies you asynchronously of the state of the execution by sending you messages such as Start, End, Break, and Trace. You can update the application user interface in response to these messages.

You can create an execution directly or indirectly. For example, you create an execution indirectly when you load a sequence file that has a load callback. Before you create an execution directly or indirectly, you must create a user interface message handler. The TestStand Engine uses user interface messages to communicate the execution states and other asynchronous information to the application.