This measurement is taken along your shoulder line between your shoulder/neck and your shoulder/armhole points.
Place the measuring tape on your shoulder line.
Measure between your shoulder/neck point (where your neck curves into your shoulder, or where the neckline of a fitted neckline garment meets your shoulder seam line) and your shoulder/armhole point (the point just before the shoulder line drops off to become the arm).
Follow the angle of your shoulder line.
Record the measurement.