I was confused on this also when I purchased my computerized machine. In doing research, I found out the following regarding stitch lengths:
1.7 = 15 stitches per inch
2.0 = 12 stitches per inch
2.5 = 10 stitches per inch
3.0 = 8 stitches per inch
4.0 = 6 stitches per inch
Hope that helps someone!
I usually piece with 2.0 and quilt at 3.5.