The ruler foot has a taller profile which provides more stability when working with rulers.  The foot also has a cutout allowing great visablity of the needle while sewing.

TO ATTACH THE RULER FOOT

1.  If another presser foot is on the mount, use the 2.5mm hex wrench to loosen the mounting screw far enough that you can slide the foot off of the mount.
2.  Slide the Ruler Foot onto the mount. The opening on the side of the foot lines up with the mount’s screw hole. Push the Ruler Foot up as far as it will go.
3.  Ensure the foot is centered over the hole in the needle plate and that the needle is centered in the foot. Then fully tighten the screw with the 2.5mm hex wrench.