The “quality of service” definitions are described here:
https://developer.apple.com/library/content/documentation/Performance/Conceptual/EnergyGuide-iOS/PrioritizeWorkWithQoS.html
It looks like the “main” thread will have a QoS class of “User-interactive”. However, just because a thread is created with a QoS of “User-interactive”, does not mean that it is the “main” thread.
You can observe this in the Xcode debugger. Put a breakpoint inside the async block and look at the Debug Navigator active thread panel. When DispatchQueue.global(qos: .userInteractive).async{} is called from the main thread, it displays with a different name than the main thread.
In general, the main thread is considered the special thread where all view-related access should be performed. If something will consume any significant time, e.g. calling a web service, compressing a file, etc., you will want to run code in a separate queue, and when the process completes, return to the main queue where you update the user interface.
Note also that when using Xcode 9 with iOS 11, a warning will be emitted when a user-interface object is accessed from a non-main thread.