The Crop processes, in the PhotoDocs processes toolbar, are designed to detect the edges of paper and crop images so that the edges of the imaged page correspond to the edges of the physical page. It is designed to help improve the results of OCR.
Example: if you place a document such as a card or a contract on a flat table and photograph it with the camera directly above it, you could use Crop Flat Pages to crop the page in PhotoDocs to the edges of the actual page.
Example: When a heavy book is opened, the pages will usually curve away from the spine. The Crop Book Pages process can account for this curvature, and dewarp and crop the page so it is flat and can be easily read by the OCR process.
Example: When a long rectangular shape such as a receipt is scanned, there may be excess space on the edges of the page that can make OCR difficult. This process is designed to remove that space when scanning narrow items.