We have been working for the last 2 years on a project in which we 
decided to use MODS and then we were confronted with the issue of 
holdings (we were new to this subject of cataloguing and MARC and 
libraries in general)

So without knowing better we decided to use our extension which goes 
more or less like this (still may need some modifications)

(Not all elements need to appear)

<modsCollection xmlns="http://www.loc.gov/mods/v3">
   <mods>
     <recordInfo>
       <recordIdentifier>AAA-BBB-CCC</recordIdentifier>
     </recordInfo>
     ...
     <extension>
       <ew:holdingsInfo
           version="1.0"
           xmlns:ew="http://kalio.net/empweb/schema/holdingsinfo/v1">

         <ew:copies>
           <ew:copy>
             <ew:copyId>IEE-12312-123</ew:copyId>
             <ew:objectCategory>NP</ew:objectCategory>
             <ew:volumeNumber>1</ew:volumeNumber>
             <ew:copyLocation>XXX</ew:copyLocation>

             <!-- Other XML from other/null namespaces -->
           </ew:copy>

           <ew:copy>
             ...
           </ew:copy>

           ...
         <ew:copies>

       </ew:holdingsInfo>
     </extension>
   <mods>
</modsCollection>


This represents all the copies/holdings of a certain 
record/title/abstract bibliographic work or whatever you want to call it.

It's a first and naïve implementation but it mostly works for our needs 
of a circulation system.  Maybe today I would have taken advantage of 
MARCXML directly into the extension...

BarZ

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