Goes along with the internal flange and internal locking band (with centering extension). The outer locking band is on the surface for more assistance, making it a stronger method. Reduction in cross-segment: The inner dimensions (width and height for rectangular ducts, diameter for round ducts) from the smoke extracting ductwork https://augustexnbp.estate-blog.com/35885807/getting-my-fire-resisting-ductwork-insulation-to-work