1. First, the representation of seamless steel tubes is outer diameter × wall thickness, as there are many wall thickness specifications for seamless steel tubes, such as for a 300mm outer diameter tube, the national standard wall thickness ranges from 7.5 to 22mm, with a total of 23 specifications.

2. "DN" is commonly used to denote PVC pipes, cast iron pipes, steel-plastic composite pipes, and galvanized steel pipes. It stands for the abbreviation of Nominal Diameter in English. The number following "DN" represents the nominal diameter, which does not indicate either the outer or inner diameter. It can be roughly understood as the "average diameter" and is not to be used for calculating or comparing sizes.

3. The OD of the dn300 welded pipe is 323.9mm, with standard wall thicknesses of 3.96mm, 4.57mm, 9.53mm, and 12.7mm, totaling four types.