This analysis is currently limited to the MARC21 bibliographic format. I suspect, however, that the techniques used here and the patterns that are revealed will be the same in the authorities format.
None of this is intended to guarantee round-trip mapping between MARC21 and any other data format. I decided early on that to insist on round-tripping for all data elements would greatly complicate the task at hand. Some round-tripping would depend on instance data (e.g. keeping track of whether the fixed field value came from an 006 or an 008), which is not in scope here.