I have two questions about the 'best' way to design an xml document.
The first is about tagging a collection of similar items as such, and
the other is about a nested structure.

I'm designing an app that can handle an arbitrary tree of a company's
organizational structure. Offices are actual physical locations, while
regions are nestable logical groupings of the offices.

When a region has office(s) as children, I've include the office tags
in <offices></offices>. However, I don't include nested regions in
<regions></regions> tags. Should I? Why or why not? I've noticed that
Michael Kay seems to always enclose repeating items in collection
tags, while other examples don't.

Here's an example.

<company>
  <region>
  <name>Northeast</name>
    <region>
      <name>New York</name>
      <offices>
        <office>
          <name>New York Office</name>
          <address>...</address>
        </office>
        <office>
          <name>Albany</name>
          <address>...</address>
        <office>
      </offices>
    </region>
    <region>
      <name>Boston</name>
    </region>
    <region>
      <name>Newark</name>
    </region>
  <region>
  <name>Gulf Coast</name>
    <region>
      <name>Dallas</name>
    </region>
    <region>
      <name>Dallas</name>
    </region>
  </region>
</company>

As far as my second question, if I should enclose any set of regions
in <regions></regions> tags, then shouldn't I include the entire tree
in <regions></regions> tags? In that case, couldn't I replace
<company>, or must the root node always be unique, even with nested
xmls?