Question

What does indexpath mean for UICollectionView’s supplementaryView or decorationView?

layoutAttributesForSupplementaryViewOfKind:atIndexPath: 
layoutAttributesForDecorationViewOfKind:atIndexPath:

Answer 1

This Documentation says :

It is up to you to decide how to use the indexPath parameter to
identify a given supplementary view. Typically, you use the
elementKind parameter to identify the type of the supplementary view
and the indexPath information to distinguish between different
instances of that view.

Same for decoration views.

In other words there is absolutely no correlation between a cell indexPath which corresponds to a position in the data source, and the indexPath of the supplementary or decoration view which corresponds to whatever you want.