The following outlines the basis for the allowable stress calculation, 0.6Fy × AMOD, used in the SACS plate Von Mises code check: 

In AISC 9th edition ASD Manual, the primary factory of safety against material yielding is 1.67. Therefore, the allowable stress is calculated as Fy/1.67 = 0.598*Fy. This value is rounded to 0.6*Fy for simplicity and general use in tension and combined stress calculations.

While the von Mises stress is a combined stress state (not just pure tension), the same fundamental principle applies. The von Mises yield criterion predicts that yielding occurs when the von Mises stress equals the yield stress (Fy). To prevent this with a factor of safety, the calculated von Mises stress must be less than the allowable stress, which is Fy/1.67 or 0.6*Fy.

The AMOD factor is an "Allowable Stress Modifier," which is a feature within the SACS software to account for load combinations where the AISC code permits an increase in allowable stresses (e.g., for load combinations including wind or seismic loads).

In summary, the reference for using 0.6*Fy as the allowable stress is rooted in the fundamental safety factor of 1.67 against yielding, as established in the AISC 9th Edition, ASD specification.