As reported by George Praeger on the AALL TS-SIS discussion list:
MARC 21 Update No. 10 (October 2009) has recently been integrated into the documentation for each of the Online Full and Concise formats that are maintained on the MARC website (www.loc.gov/marc/). Changes to the documentation resulted mainly from MARBI proposals that were approved at the ALA Midwinter and ALA Annual Meetings in 2009. These changes are indicated in red in the Online Full version of the formats.
Each format also has an appendix, “Format Changes for Update No. 10 (October 2009)” that lists all the fields with changes that are part of the update.
Some major changes to the MARC 21 Bibliographic Format are:
1. New fields 336 (Content Type), 337 (Media Type), and 338 (Carrier Type). These will replace the GMD used in field 245 if/when RDA is implemented.
2. New field 588 (Source of Description Note), which will replace the general 500 note for information such as Source of title, and Description based on notes.
3. Subfield $x (ISSN) in series statement in 490 has been made repeatable, so that ISSNs may be given for both main series and subseries in the same 490 field.
4. Field 787 has been renamed from “Other Relationship Entry” to “Nonspecific Relationship Entry.”
Some major changes to the Authority Format:
New fields: 046 (Special Coded Dates), 336 (Content Type), and a block of fields 370-377 for various new elements in authority records relating to Group 2 FRBR entities.
In the United States, the new fields in the MARC 21 Bibliographic & Authority Formats will usually not be used until the changes are implemented by OCLC, LC, and the PCC.
For more on any of these fields, refer to the MARC 21 formats, the relevant MARBI Discussion papers and Proposals, and/or my last MARBI report, available at: http://www.aallnet.org/sis/tssis/representatives/2009/marbi2009.htm