Question

I find “true to make the segment stroked when a Pen is used to render the segment; otherwise, false.” to be an useless definition.

Answer 1

If it’s set to false the line is invisible/not drawn.

using (StreamGeometryContext ctx = geometry.Open())
{
    ctx.BeginFigure(new Point(10, 10), false, true);
    ctx.LineTo(new Point(110, 110), true, false);
    ctx.LineTo(new Point(10, 110), true, false);

    ctx.BeginFigure(new Point(120, 10), false, true);
    ctx.LineTo(new Point(220, 110), true, false);
    ctx.LineTo(new Point(120, 110), false, false); // don't draw a visible line here
}

This draws a triangle and an “triangle” with only two sides visible