If I recall correctly this has been brought up several times on the list, and the consensus seems to be that "Writer of added commentary" is only used when the entity is providing an "interpretation or critical explanation of the original work". (For example: an edition of Plato's Republic with added commentary by Professor Plum.)
For prefatory material, which usually--though not always--involves commentary on the production of the text but not the text itself, there is curently no established RDA relator term (though I know there are a bunch of new terms being considered by JSC, so maybe one will appear soon?) but the MARC relator (http://www.loc.gov/marc/relators/relaterm.html) "Author of introduction, etc." is valid.
Acquisitions, Metadata and Enterprise Systems
From: Program for Cooperative Cataloging [mailto:[log in to unmask]] On Behalf Of Karen Jensen
Sent: Tuesday, July 02, 2013 11:27 AM
To: [log in to unmask]
Subject: [PCCLIST] Relationship Designators for authors of preface?
I realize that generally no access point is made for the author of a preface, but sometimes records do include the author of a preface in the 700, perhaps because an important author.
If adding a relationship designator to the 700 in this case, would it be "writer of added commentary" from I.3.1 Relationship Designators for Contributors?
When we do start adding relationship designators to our records, is it possible to leave some 700s without one?
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