Goes together with the internal flange and internal locking band (with centering extension). The outer locking band is on the surface for more help, which makes it a more robust system. Grease duct is both routed via a fire rated shaft enclosure or wrapped with the authorised fire insulating blanket https://dantejcsgv.thezenweb.com/what-does-fire-resisting-ductwork-insulation-mean-75225899