At 2.GCD vs. Operations, under Grand Central Dispatch, there is a box that says in it:
“Note: There is no guarantee as to which thread your task will execute against.”
On the other hand, on Apple’s Developer Documentation → Dispatch- > DispatchQueue, it says under Overview:
“Except for the dispatch queue representing your app’s main thread, the system makes no guarantees about which thread it uses to execute a task.”
Does that exception apply to the Note in the book I quoted above?