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A Callback Function is just a function that is automatically called at certain intervals during the execution of an action, typically so you can report on the action's progress. It's called a "callback function" because the action is said to be "calling back" to your function.
Callback Functions are mostly useful for actions that may take longer to complete. In those cases, it is nice to let the end user know what is happening so they don't start to worry. All actions that contain a callback function are automatically configured to pass the action's progress information to the built-in status dialog if the default value nil is passed as the CallbackFunction parameter. The status dialog can then be shown, hidden and manipulated using the available status dialog actions. Another purpose of the callback function is to provide the ability to interrupt the action's progress if required.
While using the status dialog may be the most common use of Callback Functions, there may be other tasks that need to be performed based on the progress of the action. In these cases you can create your own function to handle the action's progress in a custom way. Each Callback Function requires specific parameters to receive information from the action. Each action's callback parameters can be found in the action's help topic.
The following actions contain a Callback Function parameter: