It is a plexi-glass piece made to the exact dimensions of the cabinet's opening and your specific sewing machine. The insert is used only in the flatbed position of the cabinet, which is the second or middle position on the airlift. Remove your accessory kit from the front of your machine to reveal your freearm. The insert fits around the freearm and center of your machine. A Custom Machine Insert is available for many of our cabinets to ensure that you have a snug fitting platform around your machine when the cabinet is in the flatbed position. NOTE: Your Custom Machine Insert takes up to 3 weeks to be made.