What is all this New, Old and Element stuff inside a Custom renderer?

Hi all, I'm building a small library of controls that aren't provided by Xamarin.Forms like Checkboxes and RatingBars. So far they are working well, but I want to know the right way to set up the renderer for them.

Explicitly what is the use of OldElement, NewElement?

Docs only give this template, but I pesonally think that it isn't very instructive:

protected override void OnElementChanged (ElementChangedEventArgs<NativeListView> e)
{
  base.OnElementChanged (e);

  if (Control == null) {
    // Instantiate the native control and assign it to the Control property
  }

  if (e.OldElement != null) {
    // Unsubscribe from event handlers and cleanup any resources
  }

  if (e.NewElement != null) {
    // Configure the control and subscribe to event handlers
  }
}

In which scenarios are OldElement distinct from null and in which is null, and in which is NewElement null or not?

Hope someone could help me to understand.