Answering my own question for those that also need to know….. Large Reg O XL80" on a men's shirt means that the shirt is a size "Large" in terms of chest fit, with a "regular" length, and the sleeve length is specifically designed for someone with a 34" arm, indicated by the "XL80" (where "XL" signifies extra-long and "80" likely refers to the sleeve length in centimeters.