MARC-21 has the concept of relator codes and relator terms which can be associated with a contributor to indicate their role in creating an item. Here's LC's list. I'm sure there's a reason why you can use either the $e to put in a term, or $4 for a code, but from a processing point of view, that's just one more redundancy we have to handle. I recently did a scan of our July 2006 copy of WorldCat. I found 369 different codes and 9,456 terms after some normalization. Here are the top codes, along with the number of times they occurred:
and the top relator terms:
joint author (474,307)
former owner (34,892)
I was struck by the small amount of overlap between the two lists. Most of the terms and codes are self explanatory. PRF is performer and ITR is instrumentalist. It wasn't obvious what orient is, but since they occurred in Portuguese thesis records I asked my friend Ana Pavoni in Brazil, who happens to be a world expert in ETDs. She says it stands for 'orientador' which means mentor or supervisor. Former owner refers to someone that once owned the item (usually a book). It surprised me to see it occur so frequently, but for some items it is of interest.
The total number of occurrences was a bit higher than I expected, and it looks like both fields need to be inspected. I've made a table of the more frequent relator terms encountered in records and translate those into relator codes for my processing. I've found that if a person's records have a high proportion of cmp (composer) codes and a high proportion of records that indicate 'language' as zxx (non-language material), they are what most people think of as composers.