2. The 5 here refers to the thickness of the glass, which is what we usually call five centimeters of glass. The thickness is 5mm, plus or minus two. How big is the gap between the hollow glass composed of two layers of 5cm glass? It is 18mm.
3.5 18 in 18a 5 refers to the gap between two layers of insulating glass, that is, the interval is 18mm, and a is the abbreviation of air in English.