WPF DefaultStyleKey vs DefaultStyleKeyProperty.OverrideMetadata

Question

What is the difference between these

static XXXXXXX()
{
DefaultStyleKeyProperty.OverrideMetadata(typeof(XXXXXXX),
new System.Windows.FrameworkPropertyMetadata(typeof(XXXXXXX)));
}

and

public XXXXXXX()
{
DefaultStyleKey = typeof(C1XXXXXXX);
}