Density is a term that means the distance between two stitches and is the single most important factor in making embroidery digitizing look good. Too little density creates stitches that do not cover the fabric. Too much density creates bunching up of stitches, lumps of stitches in one position where the needle pounds several stitches on top of each other, breaks in the thread, broken needles.
Density settings are a measurement between rows of stitches so you can simply decide what distance you want between rows and change the setting to reflect the distance between rows. If you choose too high a stitch density you will increase the use of thread and machine time; possibly resulting in additional thread break, all of which are very expensive and also frustrating. If you choose too low a setting then “gaps” and “grins” will start to appear where the fabric color protrudes through the embroidered area.
Using a combination of underlay and cover stitching uses far less stitches than using cover stitching alone, so always use underlay if possible, especially for large fill areas. Using the correct stitch density will result in a significant saving of unnecessary stitches; an area sewn at a density of 0.5 mm uses 20% less stitches than a density of 0.4 mm, saving not only considerable amounts of machine time but also the amount of thread used.