Can also be used with seat retention devices, baggies, and other headrests. The 25-2913-00 is a unique donut shape with geltron placed in a U-shape to stabilize the head and gently support it. It can also be used to de-pressure locally such as ears. ● 25-2913-01 has a recessed in the center of the foam on the bottom of the pillow to stabilize the head.