Sewists are often confused by the terms understitching, topstitching and edgestitching.
Understitching is used to prevent edges from rolling out and showing on the right side of the garment. It keeps the edge flat and is not visible from the right side of the garment. Understitching is most often seen on the inside of the garment along necklines, faced edges around armholes, under lapels, or under collars and cuffs. To understitch a seam, first sew the seam, press the seam open, and then to one side. Grade the seam allowance.
Grading is the process of trimming the seam allowances shorter, with one seam allowance shorter than the other. When grading remember that the seam allowance that lays under the side of the garment that people will see is always the longer edge.
Now, using an edge stitching or edge joining foot with needle moved three positions to the left, feed the well of the seam alongside the metal guide of the foot. Watch the guide. Do not watch the needle.
Topstitching and edgestitching are decorative stitching, which show on the right side of the garment. Topstitching is wider, whereas edgestitching is done close to the seam. Topstitching tends to look more casual and edgestitching more dressy. To obtain the best, straightest results for topstitching try using a straight stitch throat plate and a presser foot with a single needle hole.
Sew the seam and press it open. I like to use a 1/4" foot with its small needle opening. I line up the edge of the foot with the well of the seam. Topstitch down each side of the seam. A longer stitch length of 3 - 3.5mm will make topstitching stand out.
To edgestitch a seam, sew the seam, press the seam open and then to one side. For really straight results use the same throat plate and foot as for topstitching but align the foot so that it is stitching approximately a narrow 1/8" away from the well of the seam. My 1/4" foot has a notch, which allows me to feed the well of the seam into the notch as the machine stitches the right distance away from the seam. Alternatively you can use the all purpose throat plate and an edgestitching foot with the needle moved three positions to the left.