The very edge of the pattern piece, where the fabric is cut. This outermost line is beyond the seam-line (or sewing line) and the seam allowance.
The Skirt Front & Skirt Back are shown at the top with seam allowance added.
Below is a copy of the Skirt Back & Skirt Front with the Cutting Line & Sewing line highlighted; the green line indicates the Cutting Line and the pink dashed line indicates the Seam Line ( or Stitching Line / Sewing Line).
Note: The Seam Allowance is the portion between the Stitching Line and the Cutting Line.