I often get asked in my surface embroidery classes such as needle painting what the difference between stem stitch and outline stitch is.
Stem stitch and outline stitch are the most commonly used stitches for lines on surface embroidery. They are worked in a similar method but can have different appearances depending on what thread you use. The main difference being the thickness of the line and the twist of the thread. As I mostly use just one single strand of fine DMC embroidery cotton thread in my designs it is not very noticeable to see the difference so I am not concerned if a stem stitch or an outline stitch is used but it is good to know if you are working on larger designs and using a heavy thread such as perle cotton.
Mary Corbet of NeedlenThread forum wrote this excellent article on stem stitch versus outline stitch which is well worth reading. There are also video instructions on how to work stem stitch and outline stitch if you are not familiar with the stitches.
Enjoy it is a great visual learning experience.