what's inadequacy between schema namespace an public anxiety wpf?

why there twin kinds references xaml.

one looks this:

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 

but cave feeling this:

xmlns:wpftoolkit="clr-namespace:microsoft.windows.controls;assembly=wpftoolkit"

why can't i this:

xmlns:local="http://myschema.mydomain.com/mycontrolnamespace

thanks answer. here's footnote reason