Why does SGML use zone=?

I see many places in the SGML docs that we do:

   <indexterm zone="functions-info">
    <primary>user</primary>
    <secondary>current</secondary>
   </indexterm>

In almost every case the "zone" tag has the same name as the section it
is in.  However, there are other cases that have no "zone" tag and they
default to the section they are in.  What I am wondering is why we
specify "zone" at all?  Why not just let it default to the section name?
Seems that would make writing and editing easier.

The SGML docs at:

    http://www.oasis-open.org/docbook/documentation/reference/html/indexterm.html

Say:

    Zone

        The use of Zone implies a spanning index entry. Zone holds the IDs
    of the elements to which it applies. The IndexTerm applies to the
    contents of the entire element(s) to which it points. If Zone is used,
    the phyiscal placement of the IndexTerm in the flow of the document is
    irrelavant.


It seems "zone" is for cases where you want to specify index terms
outside of the main document flow, but we don't do that.

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  Bruce Momjian  <bruce@momjian.us>          http://momjian.us
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