Swift function, two names for one parameter [duplicate]

Question

I have noticed that some methods e.g. init(nibName nibName: String?, bundle nibBundle: NSBundle?) has two "names" for one parameter except the first one is not possible to use inside method. In this case you are not able to use bundle but can use nibBundle. For example, when I call super.init(nibName: nibName, bundle: bundle) I get error "Use of unresolved identifier 'bundle'".

My question is: What is it (double named parameter) for? How to use it properly?

EDIT: It is now obvious it is External Parameter Names thing. I have subclass of UIViewController and override following method. I don't get where nibBundle gets from? Apparently it is not defined in function header.

override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: NSBundle?) {
   super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
   let someBundle = nibBundle
   print(someBundle)
}

Answer 1

First name is public name, second - private (ca be used only in function)

Answer 2

From Apple's documentation:

Sometimes it’s useful to name each parameter when you call a function, to indicate the purpose of each argument you pass to the function.

If you want users of your function to provide parameter names when they call your function, define an external parameter name for each parameter, in addition to the local parameter name. You write an external parameter name before the local parameter name it supports, separated by a space:

func someFunction(externalParameterName localParameterName: Int) {
    // function body goes here, and can use localParameterName
    // to refer to the argument value for that parameter
}

Shorthand External Parameter Names

If you want to provide an external parameter name for a function parameter, and the local parameter name is already an appropriate name to use, you do not need to write the same name twice for that parameter. Instead, write the name once, and prefix the name with a hash symbol (#). This tells Swift to use that name as both the local parameter name and the external parameter name.

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Functions.html