I don’t think LCRI 21.30G made it into RDA or a PS.  LC-PCC PS for 2.12 covers transcription aspects, and it kind of implies there would be no special treatment for entries of supplements and special numbers.

John Hostage
Senior Continuing Resources Cataloger
Harvard Library--Information and Technical Services
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Harvard Law School Library
Cambridge, MA 02138
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From: CONSER Cataloging Discussion List [mailto:[log in to unmask]] On Behalf Of Julian Everett Allgood
Sent: Wednesday, March 02, 2016 10:10
To: [log in to unmask]
Subject: Re: Questions about serial supplements

Colleagues :
Hi -- Steven's message about Supplements and Serials reminded me of a familiar AACR2 rule and Library of Congress Rule Interpretation (LCRI) that I am not positive made it into RDA. (??)
(Or perhaps I just haven't found it yet -- guidance gratefully accepted (smile!)
AACR2 Instruction 21.30G and the related LCRI section addressing "Unnumbered Supplement or Special Number to a Serial" is one that we apply often here at New York University (NYU). Because of our processing lines, when a supplement to a specific issue or volume is Received/Checked-In in the Serials Unit , the Monographs Unit often does the descriptive cataloging for the Supplement or Analytic volume.
The Serials Unit has a specific set of instructions reminding Monographs catalogers to provide an additional access point/tracing to the Serial in the form:
730  0   Algerie litterature/action. $n No 149/152 (Supplement)
.... rather than the more familiar Series added entry:

830  0   Algerie litterature/action ; $v no 149/152
So yes, as Steven, Regina, Kevin and others have pointed out, supplements to Serials get complicated quickly. They come in many shapes and sizes, and individual libraries often develop their own distinctive decision-making and workflows to process them.
I agree with Steven that it would be helpful for many catalogers if CONSER were to develop explicit processing guidance and instructions for as many of these serial supplement situations as possible.
Oh, and if anyone can point me to where AACR2 21.30G and the LCRI lives in RDA (or if it has made the RDA transition yet), I would be most appreciative. (As would all of those Monographs Copy Catalogers here at NYU I keep pointing to an old AACR2 instruction (smile!)).