XAML 2009 extends its prior version of XAML, adding a few new features.
XAML 2009 is the shorthand term for new XAML language features from .NET Framework 4.0 and later, that extend the existing XAML language specification. XAML 2009 has introduced several new directives and constructs. XAML 2009 is fully supported only in loose XAML files (those without code behind). As of now, there is no support for XAML 2009 in Visual Studio 2010 and WPF 4 unless the markup is loose XAML.