What academic discipline does "Productivity Science" come under?
There is no such thing as a


  a foolproof set of categories to put each document in


With paper documents the document had to "live" in a single locations and therefore needed a primary category/label/tag. You then had lots of different indexes that would allow you to find the document from multiple places. You often had to walk to a different library to collect the document. This system was/is inefficient and problematic.

Electronic documents can "live" in multiple places. The concept of assigning a single primary category to an electronic document is dated and silly. You should either modify the file directly to add helpful metadata or create a database that lets you externally create searchable metadata. This then allows you to easily search for the document, and related documents.