ASME B31.3 Category D Fluid Service Undercut "I"

[Xrayz417]

Any help would be greatly appreciated! I am currently making a cheat sheet for ASME Category D Fluid service and my issue is Undercut. I've read Note 6 and it explains if an "and" is part of the information, It is the lesser value that shall be taken. My question is, why would they indicate 1.5 mm if the lesser value is Tw/4 or 1 mm stated? Does this mean that based on Nominal Wall Thickness, If I used Tw/4 then 2"S40 would be 0.98 mm and 2" S80 would be 1.39 mm? Or do I only use Tw/4 if the pipe size will have a lesser value than 1 mm? and why the 1.5 mm? The interpretation is confusing....