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Calendering is a process by which fabric ran through hot, heavy rollers at the mill. This flattens the fabric, seals the weave, and leaves one side of the fabric shiny while the other is matte/dull in appearance. This process is done to make the fabric downproof, increase wind resistance, and improve hand feel.
Because of the heat involved, calendering does tend to decrease the tear strength of the fabric somewhat. So for a given fabric, there's a tradeoff between things like downproofness/wind resistance and tear strength when calendering.