When using welded pipes, depending on the weld type and testing. A certain safety factor is used.

tolerances depend on fabrication process. generally this is 12,5% for asme with SA999

In service part of the pipe can corrode away. to take this into account part of the pipe is reserved for corrosion and not calculated with

Youngs modulus at design temperature is used because under external pressure the pipe is subject to buckling

Poisson ratio is the ratio of the lateral strain to the axial strain. this is used to calculate the buckling of the pipe under external pressure

Length factor of the pipe that is unsupported LxD this is used to calculate the buckling of the pipe under external pressure