The new figure 323.2.2A replaces the previosly used note 3. If the stress ratio is 0.3 or less (note 6 ksi is 0.3 times 20 ksi, a typical allowable stress for carbon steel), which is 0.1 or less times the tensile strength, there is not sufficient strain energy for crack propogation in a brittle fracture mode. Thus, you can use carbon steel down to -104C if the stress ratio is 0.3 or less.

The figure gives temperature adjustments between the 0.3 ratio and 1.0.

The stress ratio must include all stress, including stress due to thermal contraction. However, peak stresses need not be included, thus the note that SIFs do not need to be included.

The philosophy is the same as can be found in the ASME Boiler and Pressure Vessel Code, Section VIII, and the stress ratio chart is essentially the same as can be found in Section VIII, Div 2, which also has an allowable stress based in part on 1/3 ultimate tensile strength.