Is the listing standard for type B and BW venting. It allows for a maximum rise on surface temps in the test structure of 90*F above ambient when the stabilized flue gases read 400*F above ambient for 1.5 hrs then 117*F after that. The test structure for B-vent is 3/8" plywood while BW requires 3/4". Thermocouples are placed strategically all over the box and in the vent.
Once the vent has reached equilibrium, measurements are taken at 4 circumferential points 48" off the floor inside the vent and on its outer jacket. There must be a min. 10*F temperature differential btw those readings.
If the vent is listed for a zero clearance at any point to combustibles, the outer surface of that vent cannot rise more than 90*F above ambient.