The BRITAX Infant Positioning Insert may be necessary for small infants to achieve a snug fit of the harness around your child. The Infant Positioning Insert has been tested and certified for use with the following BRITAX convertible car seats: ROUNDABOUT 55, ROUNDABOUT 55 Canada, MARATHON 70, MARATHON 65, BOULEVARD 70, BOULEVARD 65, BOULEVARD 70 CS, BOULEVARD 65 CS, ADVOCATE 70 CS, and ADVOCATE 65 CS.