Raised once per acquired image, on the thread which drives the TWAIN message loop, and it is raised synchronously - scanning of the next image continues only after the handler returns, so keep the handler short and hand the work over to another thread when it takes longer.

Order of the operations for every image: the image is added to the collector, this event is raised, the library ends the transfer (MSG_ENDXFER) and asks the device how many transfers are still pending. The device is therefore still in the transfer state while the handler runs, which is why requests which have to reach the device in a different state - like StopFeeder - are only registered here and sent by the library at the right moment.

Order of the events for the whole scanning: AcquiringEvent and OnSingleImageAcquired per image, then OnBatchFinished when there are no more pending transfers, and finally OnScanningFinished after the data source is disabled.