Use an ad hoc event when you need to include ad hoc actions that can be run at any time during process execution.
If you want to enable end users to perform an ad hoc action during the execution of another process, you can do so by using a start ad hoc event in your BPD. For example, you may want to enable end users to cancel an order, determine the status of an order, or perform some other ad hoc function during the normal processing of an order. Because an ad hoc action is run in the context of the regular process instance, it has access to all the data of the regular process instance and can also manipulate the flow of the regular process instance, depending on the logic that you include.