Low - P gives the fabricator less margin for error, the assembler, more margin. Hgih P is the opposite. But the margin on low P is VERY thin. The change to mid-P is a big reason why black pad has become far less prevalent. I would not accept <7% anymore
Higher P causes a near self limiting gold corrosion, keeping the gold from corroding the Nickel (black pad). But it does concentrate the P near the Ni/Sn Intermetallic formation of the joint, making dark line defects (brittle P layer) easier to create during reflow. High P also allows thinner gold, less to dissolve into the IMC.
Pete