What is "Item Classification"

I have been using Upload Templates for about 2 years successfully.
However, when I recently changed the Sizing to meet the new Guidelines I got the message “You cannot change the item_classification”, for quite a few of the Parent SKUs.
Amazon “Support” just told me to use a different SKU for the parent and Child but this is obvious and has always been the same:- Parent 01-00012 and Child 01-00012-001, for example.

Can anyone tell me what the “item_classification” is and what this message means, as there is no Field called this.