I have a question about PCC practice for the URLs for internet resources 
that are no longer available.

LRI 9.7B says the following

2) If searching indicates that the resource is no longer available, 
create a note to reflect this fact by changing subfield $u in field 856 
to subfield $z and modifying the subfield to show that the resource is 
no longer available, indicating the last date that the resource was 
searched. ...

and gives the example
revised record
856 41 $z Electronic address ( not available when 
searched on [date]

However CONSER cataloging module module 31 (dated March 2012) says:

If the only link appearing on the CONSER record is an invalid link, it 
can be left on the record and labeled as invalid in the subfield $z of 
the 856 field. Note that the second indicator is blank and that the 
non-working URL is maintained in subfield $u of the 856. This coding 
differs from LC practice documented in LCRI 9.7B where the non-working 
URL is moved to a subfield z so that it does not appear on LC’s link 
checking reports repeatedly. The example below is based on a 
recommendation from OCLC and is derived from current system indexing 
needs and OCLC'€™s electronic address checking software (see OCLC'€™s 
recommendation at:
856 4# $z Link no longer valid as of Dec. 4, 2000 $u http://www...

So, we're confused about which practice to follow, since it seems that 
there are two conflicting PCC practices. This is also causing us 
problems internally in our opac, since we can't suppress the display of 
a hotlink if the URL is in $u. And we don't quite understand why one 
would use a public rather than non-public note ($x) for this.

Greta de Groat
Stanford University Libraries