When to initialize superClass properties and when to initialize self properties?

So I wanted to take the exercise a step further and make the numberOfSeats a variable and set it to a default of 4 seats. So I go about making my subclass like this.

class Car: Vehicle { var numberOfSeats: Int

init(withDoors doors: Int, andWheels wheels: Int, andSeats seats: Int = 4) {
    self.numberOfSeats = seats
    super.init(withDoors: doors, andWheels: wheels)

}

}

Which seems to work. The only problem is that I originally had the two lines within the initializer flipped and Xcode was giving me an error saying "Property 'self.numberOfSeats' not initialized at super.init call." I don't understand why this works. I though you need to have super.init called first before self.property. Can someone explain why self.numberOfSeats needs to be initialized before the super?