What is the purpose for ItemInstance?

["G. Ken Holman" <gkholman@CraneSoftwrights.com>]

Hi folks!

In an ongoing development, I've been told my earlier use of 
ItemInstance was inappropriate to describe additional properties of 
an item.  The latest version of an invoice I'm working on is here:

   http://lists.oasis-open.org/archives/ubl-dev/200804/msg00007.html

The UBL description for <cac:ItemInstance> reads "An association to 
Item Instance" .... which tells me nothing about its use.

What is the purpose of <cac:ItemInstance>?  Rewriting the description 
could go on to a change list for a future version.

When would I use <cac:ItemInstance> or when would I use 
<cac:AdditionalItemProperty> as a child of <cac:ItemInstance> instead 
of as a child of <cac:Item>?

Thanks for your thoughts!

. . . . . . . . . . . Ken

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