The term “Categories” as used in Section VIII (Rules for Construction of Pressure Vessels), Division I, Subsection B, Part UW-3 specify special requirements regarding joint type and degree of inspection for certain welded pressure joints. Since these special requirements , which are based on service, material, and thickness, do not apply to every weld joint, only those joints to which special requirements apply are included in the categories. These categories are:a)Category A. Longitudinal welded joints within the main shell, communicating chambers, transitions in diameter, or nozzles; any welded joint within a sphere, within a formed or flat head, or within the side plates of a flat-sided vessel; circumferential welded joints connecting hemispherical heads to main shells, to transitions in diameters, to nozzles, or to communicating chambers.b)Category B. Circumferential welded joints within the main shell, communicating chambers, nozzles, or transitions in diameter including joints between the transition and a cylinder at either the large or small end; circumferential welded joints connecting formed heads other than hemispherical to main shells, to transition in diameter, to nozzles, or to communicating chambers.c)Category C. Welded joints connecting flanges, Van Stone laps, tubesheets, or flat heads to main shell, to formed heads, to transitions in diameter, to nozzles, or to communicating chambers any welded joint connecting one side plate to another side plate or a flat-sided vessel.d)Category D. Welded joints connecting communicating chambers or nozzles to main shells, to spheres, to transitions in diameter, to heads, or to flat-sided vessels, and those joints connecting nozzles to communicating chambers.