The documentation for NSCollectionViewItem refer a Collection View Programming Guide for macOS.
"Legacy Item Support
For apps built before OS X 10.11, you created a template item and assigned it to the itemPrototype property of your collection view. To create new instances of the item, you called the collection view’s newItemForRepresentedObject: method. For more information about how to support older collection view configurations, see Collection View Programming Guide for macOS."
Where can I find this Collection View Programming Guide for macOS?