An object used to add headers or footers to a collection view.
Mac Catalyst 13.0+
A boundary supplementary item is a specialized type of supplementary item (NSCollectionLayoutSupplementaryItem). You use boundary supplementary items to add headers or footers to a section of a collection view or the entire collection view.
Each type of supplementary item must have a unique element kind. Consider tracking these strings together in a way that makes it straightforward to identify each element.