Most likely it controls the pressure of your presser foot. But that is just a guess. You will have to experiment to see if the higher numbers are for thicker fabrics. You would probably use it more if garment sewing to allow for different thicknesses of fabrics. With piecing, because most quilting cottons are the same thickness you probably won't adjust it much. And for FMQ you would be using a different foot and dropping the feed dogs so it would be irrelevent. You may want to adjust it for quilting with a walking foot though.
Edited to add that you can purchase the manual from Singer but it costs $15
http://www.singerco.com/accessories/...manuals/search