The MODS Editorial Committee has discussed this in the context of the suggestion that we be able to show the missing information for authors (i.e. "et al" as specified in this email and previous ones):

We have come up with the following proposal to accommodate this need.

Proposal:  define element <etal>, subelement of <name>
<name> currently has the following subelements:
<etal> would be added to the list of <name> subelements, to indicate that there are one or more names that, for whatever reason, cannot be explicitily included.
<etal> could occur as:
o    an empty element, as in <etal/>, or
o    with simple content, as in for example <etal>et al.</etal>; in this case the content is what is suggested for display.
When <etal> occurs (either empty or with simple content):
         <namePart> MAY NOT occur
         <displayForm> MAY NOT occur.
         <affiliation>, <role>, <description>   MAY occur (but are not repeatable). 
<etal> is not repeatable within a given <name>, however there may be mutilple <etal> elements, each within in a separate <name> element.

author=(Scheffler, Thomas and others)

	<namePart>Scheffler, Thomas</namePart>
	<etal>and others</etal>

-or- (if you don't want to put in a value for display):

	<namePart>Scheffler, Thomas</namePart>
We propose to add this to version 3.5, which is almost ready to go out for public review. The Editorial Committee will consider additional changes in the next version to accommodate the other types of situations where there is missing information, as detailed in Ben's message below.


Rebecca Guenther
Library of Congress
Network Development & MARC Standards Office
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On Jan 10, 2013, at 9:19 AM, Ben Companjen wrote:

> Hi,
> In December I asked the question in the subject on the Libraries and
> Information Science StackExchange:
> "In many MODS records harvested from several sources, I see
> <mods:publisher>s.n.</mods:publisher> or something similar (worst I
> have seen: 000NOPUBLISHER000) to indicate the publisher is unknown or
> that there is no 'official' publisher. For unknown information, I
> think leaving elements out makes sense. However, for information known
> to be missing, something else may work better.
> Are there special field values (perhaps "unknown") or structural
> elements (perhaps 'unknown="true"') to indicate 'this value is known
> to be missing'?"
> <>
> But later I realised this list is more appropriate for my question. I
> think I read most of the guidelines and I searched the recent archives
> and found Thomas Scheffler's question and following discussion on "et
> al." and "and others" and a quote by Nathan Humpal "... using s.l., or
> any <place> element at all, if the place name is not known is advised
> against". There was an exchange in July 2003 about qualifiers for
> 'knownness' of elements like names and dates, but it seems they (e.g.
> @qualifier="unknown") have not made it to the schema definitions.
> Can I say that in general, there is no way of saying part of a record
> is unknown other than by leaving it out and perhaps making a mods:note
> of it? And (if it is indeed semantically different) if some
> information is known to be missing (e.g. the 'et al.' case)?
> Thanks in advance. (Perhaps I should add that I'm not a cataloguer or
> librarian in general. :-))
> Ben Companjen
> P.S. Feel free to answer or comment on the LIS SE.