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The Magento experience has been frustrating… I called Magento sales with a different question but only got their answering service. They never bothered to return my call…
Back to this topic, I did not find a list of category IDs anywhere, but I did find the IDs listed in the admin console. If you click on each category level the ID number appears at the top of the page. (See attached.)
I’m posting it here in case it helps someone else.