This information is from Superior Threads. An excellent illustration of tension.
If the threads are identical and you are sewing on a single layer of fabric and the tension settings are perfect so both sides have equal strength, the result will be a draw. The sewing should therefore produce perfectly even stitches with no top thread showing underneath and no bobbin thread showing on top. However, in the real world, the teams are rarely equal. One team will be stronger or bigger or faster or smoother than the other. We sometimes use decorative or sensitive threads on top. We often use different fibers for the top and bottom threads. We also add stabilizer or batting. Sometimes we might use a cotton bobbin thread and other times we use a polyester bobbin thread. All these factors make it necessary to adjust the tension for each project. By adjusting the top tension either up or down, we are able to add or take away strength on the top thread team to equalize the tug of war battle. Following is a list of things that affect stitch results:
1. Batting. This adds drag on the top thread. Cotton batting tends to grab the thread more than poly batting, adding more friction on the thread.
2. Fabric type. Dense fabric puts more stress on the thread.
3. Top thread thickness and type. Metallic is less flexible than cotton or poly. Poly is usually stronger than cotton or rayon.
4. Bobbin thread type. Cotton bobbin thread tends to grab more than a smooth filament polyester. Sometimes grabbing is preferred (when piecing) and sometimes it causes problems (with metallic thread). A smooth filament poly thread (not spun poly) in the bobbin will work better with metallic and other sensitive threads because its smooth finish acts almost like a lubricant, sliding nicely with the thread.
Conclusion: We cannot rely on automatic tension settings. There are too many variables. As illustrated in the diagram (click on link below), if the top thread is showing underneath, either the top tension is too lose or the bobbin tension is too tight. Either tighten the top or loosen the bobbin. If the bobbin thread is poking through the top, either the top is too tight or the bobbin is too loose. Loosen the top or tighten the bobbin. When making adjustments, begin with the top tension.