Boy have you bitten off a mouthful with that question! I have been trying to resolve that one myself, and have contacted various structural engineers and both AWS and AISC. Still no clear answer so far. I even have started to draft a technical inquiry for the D1 Commitee, so if you get a definitive answer, please, please, please, share it with everyone!

The nearest I can tell you is that most everyone I talked to feels a re-entrant corner is a type of a cope or clearance cut in structural members. This could be a transition in section depth of a beam or girder.
Everyone agreed that D1.1 clearly addresses reentrant corners and beam copes differently. That reentrant corners require a 1" radius while a beam cope only requires a smooth transition free of notches.

Where I first butted heads on this one is when a QA inspector insisted that we provide 1" radii (2" diameter) in our copes because he felt copes were reentrant corners. The owner agreed with him so we had to do a lot of rework that I felt was unnecessary.

All that is water under the bridge now but I still would like to know the difference if you find out. Thanks.

CHGuilford