28. The non-transitory computer readable storage medium of claim 26 , wherein the respective subset of the document identification tags associated with each tier monotonically increase 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.
29. The non-transitory computer readable storage medium of claim 28 , wherein the instructions for assigning a document identification tag to the new document comprise instructions for assigning to the new document a minimum available document identification tag from the respective subset of document identification tags associated with the selected tier.