You can’t set World Records at the Boston Marathon, but you can set your personal best anywhere. There are no rules for personal best, unless you arbitrarily set your own rules for your PB. Some people have downhill PB’s, Trail PB’s, Road PB’s, which I think is fine to keep you motivated to do your best on race day and put positive spin on your performances. Personal Best times are yours. The B.A.A., IAAF, and not even Let’s Run can steal them.
+1. People get to decide what they consider their PR/PB. Although common courtesy calls them to give others a heads up if it’s at Boston in a tailwind year, at a Revel (downhill) race, etc.