The walking foot is used for SITD, channel quilting (straight lines) or gentle curves. Check with the manufacurer, but many are made to stitch forward only. The feed dogs move the fabric from below and the grippers on the foot move the fabric from the top, so thick layers are pulled evenly from top and bottom. The foot is also called an even feed foot for that reason.
FMQ is where you don't rely on the feed dogs to move the fabric, you do. you use a hopping foot, or darning foot .