Phil Middlemiss continues his series on creating a Chrome and Glass style theme, this time looking at the ProgressBar and Slider controls. He takes a close look at the Indeterminate State of the ProgressBar control and explains what Control Parts are.
The red circle is around the Control Part icon. That icon indicates that the control expects there to be an element in the template called "ProgressBarTrack", and it usually has to be a control of the same type as is used in the default control template - in this case a Border control for "ProgressBarTrack" and a Rectangle element for "ProgressBarIndicator".