The Edge Joining Foot (also known as the Stitch in the Ditch foot) can be used for edge stitching, sewing fabrics together side by side or for stitching in the ditch.
The vertical blade-like protrusion at the front of the foot is used to guide fabric edges when sewing. For example when edge stitching line up the end of your fabric with the blade, or when you are joining two pieces of fabric together. Simply line up each piece of fabric on either side of this blade, and it will guide your fabric through. When Stitching two pieces of fabric together side by side, ensure you are using a wide stitch e.g. Zig Zag what will catch both pieces of fabric as you are sewing.