Thanks for finding the error-- it looks like there is an error in the Cataloger's Desktop version of the MARC 21 Format for Authority Data-- in the 670$u section the 670 is erroneously tagged as 678. The print version of the authorities format has the example correctly tagged as 670. Bruce Johnson in CDS reports that the fix to Cataloger's Desktop will be distributed with 2006 Issue 2 on roughly May 1st.
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Please note that the example provided to illustrate the use of subfield $u (Uniform Resource Identifier) appears to be erronesouly tagged 678 instead of 670:
678 ## $aThe official Stephen King web presence, viewed Oct. 26, 2001: $bThe man (Stephen Edwin King; b. Portland, Maine, 1947) $u<http://www.stephenking.com>
The Stephen King Web site has been cited to document his place and date of birth, which is the kind of information normally included in field 670 for a personal name heading. Although field 678 also may be used to provide biographical data, I have not seen this field currently used in authority records created for personal names.
The example provided to illustrate the use of subfield $u in field 678 (Biographical of Historical Data) clearly is in keeping with the use of that field for providing detailed background information not normally included in field 670. The latter is used to justify heading forms or variants, as well as to provide data that form part of the heading/reference string, such as life dates or titles.