These phenomena are as follows:
When Scan Ahead is ON, the last page of the batch of documents being scanned is not saved.
When Scan Ahead is ON, the length of the front image on the next document is applied to the length of the back image of the previous document.
However, this does not occur if the length of the next document is longer than the length of the previous document.
When Scan Ahead is OFF, only the front image of the first document is saved and scanning stops.
This phenomenon could result in the loss of data should you discard the source documents after scanning.