The 'stitch line' is, quite literally, the line that you stitch along when sewing pieces together. The edge of the pattern is the line that surrounds the stitch line, once seam allowance has been added to a pattern.
When pattern cutting, it is always really important to know whether your pattern has seam allowance or not, and when taking measurements, to be certain that you are measuring the stitching line, and not the edge of the pattern,
Before cutting fabric, it is always very important to check that your patterns fit together accurately. To do this, you will need to place the pieces together, as if you were sewing them, and then check that the stitching line is the same length.