Typically this is done when a sharp corner dimension is needed. If you have a radius corner that is shown on a drawing that is dimensioned to the invisible sharp you can create this as a point in a sketch or on a drawing. Using ctrl select both edges that would form the corner. While both edges are selected use the sketch point tool. To change the virtual sharp corner display, go to tools, options, document properties, virtual sharps.
John Van Engen
CATI Tech Support