Is this the section from the book to which you are referring:
"NOTE: The PR set is not for max reps, rather it is for a rep PR. Whether or not you do a PR set or a single is 100% up to you."
Wendler, Jim (2013-07-05). Beyond 5/3/1: Simple Training for Extraordinary Results (Kindle Location 833). Jim Wendler LLC. Kindle Edition.
If it is, I was always confused by that, also. I took it to mean that if my old PR at my TM was 1 rep, I would do 2 reps (setting a PR), but not go for 3 or 4 if I was able to do so. I'm completely unqualified to know, though, and I didn't worry about it too much since I was usually working with singles.