Potential is calculated similarly to salary (although the calculations are separate).
The cap works like this. First, you multiply the skills by the numbers in the following table and take the maximum:
Role/weight JS JR OD HA DR PA IS ID RB SB
PG 0.18 0.3 0.3 0.23 0.11 0.5 0.05 0.05 0.2 0.03
SG 0.45 0.45 0.4 0.06 0.06 0.07 0.06 0.1 0.25 0.03
SF 0.6 0.25 0.3 0.05 0.05 0.03 0.1 0.2 0.35 0.03
PF 0.34 0.06 0.05 0.05 0.05 0.03 0.4 0.4 0.4 0.18
C 0.08 0.15 0 0.03 0.03 0.03 0.46 0.42 0.45 0.25
Then you check the number against the numbers in the table below:
Pot_Number Pot_Name Min Max
0 announcer 10 10
1 bench warmer 10 12
2 role player 12 14
3 6th man 14 16
4 starter 16 18
5 star 18 20
6 allstar 20 22
7 perennial allstar 22 24
8 superstar 24 26
9 MVP 26 28
10 hall of famer 28 30
11 all-time great 30 50
The minimum equals to no Potential sublevel and the max equals .99 sublevel. In between those numbers your player will cap. So let's say your player is an MVP and his real Potential is 9.50. He will cap when he passes 27 (half point between 26 and 28) in any of the cap formulas.
After he hits the cap, you can still train him, but training will slow down significantly.