Every item within HVAC conforms to a building code/standard. The pressure tables are how you can specify the different ways of constructing and reinforcing items to conform to these standards. Each pressure table can specify different “standard lengths” (different lengths for their duct). In each of these standard lengths, you can specify what metal gauge, joints, and other reinforcements for the different widths or depths of duct (or fittings).
Other topics discussing Pressure Tables include: