Let 'em shudder. Of course, there should be some more context to the question. Let's say hypothetically that an engineer worked in a refinery built in the 1960's, prior to the introduction of Div. 2. A (chemical) reactor was built and stamped to Section III to achieve cost savings similar to what would later be available through Div. 2. A crack was detected decades later in the weld overlay of a flange. Would the engineer be dealing with a cracked nuclear reactor? Imagine the fun he could have with the PR! The point is, it is possible to have Section III vessels in petrochem service. Keep an eye out for a more detailed story in the 2003 Pressure Vessel and Piping Conference volumes.
To answer your question:
From a design philosophy standpoint, Section III is similar to Section VIII Division 2. At the risk of over simplification, Div. 1 uses less engineering (one dimensional stresses), lower requirements for quality control, and more material. Section III and VIII-2 will require more thorough engineering (three dimensional stresses - Stress Intensities (aka Tresca) (soon to be Von Mises)), higher quality control of materials and fabrication, and less material.
