17. The system of claim 16 wherein the set of descriptors comprise: annotation descriptors that annotate content elements found in the POD in order to identify basic pieces of information that constitute the attributes of the objects to extract; and object descriptors comprising object schemas that define which attributes defined by at least one of the annotation descriptors and other object descriptors, compose each of the objects to extract, and a set of semantic and spatial constraints and preferences that define expected spatial arrangement of the attributes defined in the object descriptors.
22. The system of claim 17 , wherein the object instances are created by searching for annotated content elements in the SSDM that compose given objects as defined by the object descriptors.