How the T3D loads a Gui?

I have found the function that loads a Gui.
But I don't understand how the function works.

//location:guiEditorCanvas.ed.cs

function GuiEditCanvas::load( %this, %filename )
{
%newRedefineBehavior = "replaceExisting";
if( isDefined( "$GuiEditor::loadRedefineBehavior" ) )
{
// This trick allows to choose different redefineBehaviors when loading
// GUIs. This is useful, for example, when loading GUIs that would lead to
// problems when loading with their correct names because script behavior
// would immediately attach.
//
// This allows to also edit the GUI editor's own GUI inside itself.

%newRedefineBehavior = $GuiEditor::loadRedefineBehavior;
}

// Allow stomping objects while exec'ing the GUI file as we want to
// pull the file's objects even if we have another version of the GUI
// already loaded.

%oldRedefineBehavior = $Con::redefineBehavior;
$Con::redefineBehavior = %newRedefineBehavior;

// Load up the gui.
exec( %fileName );

$Con::redefineBehavior = %oldRedefineBehavior;

// The GUI file should have contained a GUIControl which should now be in the instant
// group. And, it should be the only thing in the group.
if( !isObject( %guiContent ) )
{
MessageBox( getEngineName(),
"You have loaded a Gui file that was created before this version. It has been loaded but you must open it manually from the content list dropdown",
"Ok", "Information" );
return 0;
}

GuiEditor.openForEditing( %guiContent );

GuiEditorStatusBar.print( "Loaded '" @ %filename @ "'" );
}

What is the meaning of "newRedefineBehavior","replaceExisting" and "loadRedefineBehavior"? And how the
function works as a whole?
At last,will the T3D make backup copy file for the Gui file?