A style line is a line (or curve) in a garment that has a visual effect, e.g., the seam between two fabrics of different colors. For comparison, a nearly invisible seam (say, in a French dart) would not be considered a style line. A style line is a boundary between two distinguishable areas of fabric.
As the term is generally used in practice, a "style line" is introduced for fashion only. However, a style line may also fulfil a shaping function, e.g., as a dart, as a seam at which a dart may end or as a way of hiding detail(s) of the garment's construction.