18. The system of claim 17 , wherein: the feature is a feature that was previously determined to be a feature capable of distinguishing the document image or a document that comprises the document image, and the feature was previously identified by analysis of a plurality of training documents each having at least one unique feature different from at least one of the other training documents.
18. The system of claim 17 , wherein: the feature is a feature that was previously determined to be a feature capable of distinguishing the document image or a document that comprises the document image, and the feature was previously identified by analysis of a plurality of training documents each having at least one unique feature different from at least one of the other training documents. 22. The system of claim 18 , wherein: the feature is predefined, and to identify the feature in the document image, the processor is further to perform optical character recognition (OCR) on the feature in the document image.