Tinbender is correct: Exhaust Gas Temperatures.
This is particularly important on turbocharged engines because too high temps can not only damage the turbo, but it will coke (cook) the engine oil (through the turbo) and cause bearing problems. Not sure what Ford specs are, but I would not exceed 1,000°F for any length of time. If you have a boost gauge I wouldn't exceed 35 psig for over two minutes. I suggest you change your engine oil and filter at the first opportunity. My 6.4 has compound turbo's too, and I keep a close eye on the boost gauge when towing.
