How To Measure Rectangular Duct Abbreviations S/D – Slips & Drives Connection. 4S – 4 Slips Connection. Raw – No Connection System. 1″FL Out – 1 Inch Flange Out.1″FL In – 1 Inch Flange In. DM – Duct Mate Connection.TDC – 4 Corners Bolts Connection.WMS – Wire Mesh Screen.V/D – Volume Damper.CL – Center Line A/D – Access Door.AL – Acoustical lining.FO – Flat Oval.Ø – Round.BF – Bottom Flat. BD – Bottom Down.BU – Bottom Up.TF – Top Flat.TD – Top Down.TU – Top Up