Hello Mark,

> <items>
>    <item>
>        <PlantNo>1001</PlantNo>
>        <ItemID>xx123</ItemID>
>    </item>
>    <item><PlantNo>1001</PlantNo><ItemID>xy132</ItemID></Item>
>    <item><PlantNo>1012</PlantNo><ItemID>xz321</ItemID></Item>
>    ...
> </items>
> <things>
>    <thing>
>        <PlantNo>1001</PlantNo>
>        <thingID>123</thingID>
>    </thing>
>    <thing><PlantNo>1001</PlantNo><thingID>234</thingID></thing>
>    <thing><PlantNo>1001</PlantNo><thingID>345</thingID></thing>
>    <thing><PlantNo>1001</PlantNo><thingID>456</thingID></thing>
>    <thing><PlantNo>1012</PlantNo><thingID>233</thingID></thing>
>    <thing><PlantNo>1012</PlantNo><thingID>334</thingID></thing>
>    ...
> </things>

in my opinion there are two problems with that structure:
1) You have to do a kind of mixed stepping through your structure. Because 
you want to have displayed
PLANT 1001 Items: xx123, xy132 Things: 123, 234, 345, 456
you first need to step through the item nodes and than step through the 
thing nodes while finding the appropriate PlantNo. This seems quite 
difficult.
2) You said the rule is that PlantNos are identical for things and items. 
But I think that this may not be REALLY guaranteed. So if there is some 
PlantNo missing either in the items or in the things node because of any 
reason, you would get a totally false and mixed display of items and things 
that don't belong together or wrong assignments of items/things to a plant.

As a whole I suggest to use another structure, which is much more easier to 
be stepped through.
It looks like as follows:

<plant no="1001">
 <items>
  <item id="xx123"></item>
  <item id="xy132"></item>
 </items>
 <things>
  <thing id="123"></thing>
  <thing id="234"></thing>
  <thing id="345"></thing>
  <thing id="456"></thing>
 </things>
</plant>
<plant no="1012">
 <items>
  <item id="xz321"></item>
 </items>
 <things>
  <thing id="233"></thing>
  <thing id="334"></thing>
 </things>
</plant>

The question is, if your application can supply you with that structure 
alternatively. In that structure each PlantNo only exist once so that any 
mixtures of inappropriate items and things and wrong assignments of 
items/things to a plant is impossible. Here you can step through each plant 
one after another and first list all items and second all things.

Hope this helps you.

Nice greetings from
nontoxic