So I fly a proficiency test with an instructor.
If you mean the one hour out of twelve that you need to fly with an instructor for your revalidation by experience, you have to log it as a lesson (P/UT, which goes in the Dual column), otherwise it is not counted as a lesson and cannot be used for your revalidation by experience. The instructor logs P1 and the instructor needs to countersign your logbook.
If it's a club check or something else, then you have to decide up front who logs P1. If you log P1, the instructor logs nothing. If the instructor logs P1, you can log P/UT.
At least, that's how I understand it.