It's important to return from the TransportSetItemOutput event quickly. For portability reasons it's a bad idea to prompt an operator for input during this event. If you need to prompt the operator for input, register for Ranger's ItemSuspended event then call SuspendItem() in your SetItemOutput event and return from the SetOutputEvent. 

When the check scanner is in a state where the item can be stopped, Ranger will fire the ItemSuspended event. You can then prompt the user for input and set any values for the item that you would normally set in the SetItemOutput event, such as the target pocket number. When you are ready to resume processing call Ranger's ContinueItem() call.