Structural stresses:

Structural stresses are stresses generates in structural members since of the weights they support. The weights give the loadings.  Those stresses are found for building foundations and frameworks, as well as within machinery parts.

Pressure stresses

Pressure stresses are stresses induced in vessels holding pressurized materials. The loading is generated through the similar force producing the pressure.  Within a reactor facility, the reactor vessel is a prime instance of a pressure vessel.