x:Reference Markup Extension
References an instance that is declared elsewhere in XAML markup. The reference refers to an element's x:Name.
The x:Name value (or value of the RuntimeNamePropertyAttribute-identified property) of the referenced instance.
x:Reference provides XAML language-level support for an element reference concept that was otherwise implemented in specific frameworks such as WPF.
x:Reference and WPF
In WPF and XAML 2006, element references are addressed by the framework-level feature of ElementName binding. For most WPF applications and scenarios, ElementName binding should still be used. Exceptions to this general guidance might include cases where there are data context or other scoping considerations that make data binding impractical and where markup compilation is not involved.
x:Reference is a construct defined in XAML 2009. In WPF, you can use XAML 2009 features, but only for XAML that is not WPF markup-compiled. Markup-compiled XAML and the BAML form of XAML do not currently support the XAML 2009 language keywords and features.