Let's say you've played 10 rounds.
On hole 1, your best round was round 3. Let's say, 2 feet.
On hole 2, your best round was round 7. Let's say, 11 inches.
On hole 3, your best round was round 9. Let's say, 1 foot.
Your "potential best" combines all these best results on the individual holes, but from various rounds, into one figure. So, Tibbets's "potential best" at Kiawah (I think) is zero, because he's managed to ace each of the 9 holes at one point or another.