That compartment is where you insert plastic cams to do decorative stitching. To insert the cams, put the needle in it's highest position, slide the stitch width control all the way to the right, put the cam in the top and be sure the cam sets firmly over the little locator pin in there. Once it's in, you slide the stitch width back to the left. After that, set your stich length to about 1/2, and make sure the front control knob is on "C" (except for cam number "0", that setting is on "B") . You may have to loosen the top tension a bit, so cloth doesn't pucker up. Each cam make a different pattern for top-stitching, embroidery, etc. When you are done with that cam, put the needle in it's highest position again, slide the stitch width control all the way to the right, pull out the cam, and slide the width control back to the left. Hope this helps!