First, I've been to the FAQ site and read each of the entries of Flat
File to Hierarchical. I didn't find anything that quite addressed my
problem.

My source data is flat but properly sequenced. There are two elements
whose values, if identical, imply parent/child relationship --
DocumentItemID and ParentID.

Input XML:
<Items>
<Item> (should correspond to Items/Item)
 <DocumentItemID>aaaaa</DocumentItemID>
 <ParentID>0000</ParentID>
 ...
</Item>
<Note> (should correspond to Items/Item/Note)
 <DocumentItemID>11111</DocumentItemID>
 <ParentID>aaaaa</ParentID>
 ...
</Note>
<Item> (should correspond to Items/Item/Item)
 <DocumentItemID>bbbbb</DocumentItemID>
 <ParentID>aaaaa</ParentID>
 ...
</Item>
<Item> (should correspond to Items/Item/Item/Item)
 <DocumentItemID>ccccc</DocumentItemID>
 <ParentID>bbbbb</ParentID>
 ...
</Item>

Output XML:
<Item1>
 <DocumentItemID>aaaaa</DocumentItemID>
 <ParentID>0000</ParentID>
 ...
 <Note>
   <DocumentItemID>11111</DocumentItemID>
   <ParentID>aaaaa</ParentID>
   ...
 </Note>
 <Item2>
   <DocumentItemID>bbbbb</DocumentItemID>
   <ParentID>aaaaa</ParentID>
   ...
   <Item3>
     <DocumentItemID>ccccc</DocumentItemID>
     <ParentID>bbbbb</ParentID>
     ...
   </Item3>
 </Item2>
</Item1>
</Items>

There can be as many as 8 levels of Item. There are 4 other elements
that are similar to Note and must always be a child of an Item(1-8)
element. My thought was that I could capture the Item/DocumentItemID
each time I encounter an Item element, and then each time I encounter
a Note or other child element of Item compare its child ParentID to
the Item/DocumentID and if they match output the child elements ...

I keep either suppressing output of everything but the Item elements
or repeating the Note elements in each instance of an Item element ...