20. The system of claim 18 , wherein the respective subsets of the set of document identification tags associated with each tier monotonically increases with the position of the tier in the ordering; and wherein the respective predetermined range of metric values associated with each tier monotonically increases with the position of the tier in the ordering.
20. The system of claim 18 , wherein the respective subsets of the set of document identification tags associated with each tier monotonically increases with the position of the tier in the ordering; and wherein the respective predetermined range of metric values associated with each tier monotonically increases with the position of the tier in the ordering. 21. The system of claim 20 , wherein the tag assignment module is configured to assign a document identification tag to the new document by assigning to the new document a minimum available document identification tag from the respective subset of document identification tags associated with the selected tier.