Scott;
Further on this issue, after careful study of ASME B16.5 its not really that simple.
The greater than 10,000psi answer I gave you was specific to my own project.
If you look at B16.5, it defines ratings by alloy group, pressure and temperature of use.
For "High Pressure" in B31.3, its defined as those pressures that exceed Class 2500 values.
In my case, we are using a 2-1/4 Chrome, 1 Mo alloy and our pressures are somewhere around 14,000 psi. In this instance, at the lowest temperature of use, B16.5 shows Class 2500 rated at about 6,236psi for my specific alloy so clearly we fall into the high pressure category.
Thats the problem of giving "off the cuff" answers! :)