Views — only views can have a code behind.
Behind — a view. The code exists Behind a view
*.cs sits behind *.xaml — code behind is a someName.xaml.cs file, behind a someName.xaml file
fields — defined in the xaml are directly accessible as Instance fields of the code behind class.
Part of the view — the code behind class is part of the view, not the VM or M.
event registration — often registered in xaml or (programmatically) in the code behind.