in the book this was mentioned
“Notice that headerView is a strong property. This is because it will be a top-level object in the XIB file; you use weak references for objects that are owned (directly or indirectly) by the top-level objects”
Excerpt From: Keur, Christian. “iOS Programming: The Big Nerd Ranch Guide (4th Edition) (Big Nerd Ranch Guides).” iBooks.
what does top-level object in the XIB file mean? how do you know that the headerView property will be the top-level object in XIB file?