What's guaranteed in the output order? (serial vs concurrent)

Middle PRO

Task

What is guaranteed about the output order of each snippet, and what is not?

let serial = DispatchQueue(label: "serial")
serial.async { print("A") }
serial.async { print("B") }

let conc = DispatchQueue(label: "conc", attributes: .concurrent)
conc.async { print("1") }
conc.async { print("2") }