Grading the arm from the outfield...
velocity and carry.
body control and arm action.
release time.
accuracy.
The easiest of those to improve is from bottom to the top.
Also, keep in mind that the throws are based on time from release to target. The perfectly accurate throw isn't any good if it can't get to the target on time.
A good example would be two catchers. If both catchers were receiving pitchers with 1.5 release to pop times and one catcher was 1.9 pop time but 9 out of 10 throws were inaccurate, while the other was 2.2 with 9 out of 10 right on the bag. Which one would throw out more college or pro base runners attempting to steal at this time.
That would be the catcher that wasn't accurate. Reason being he would throw out at least one out of ten baserunners. The catcher with the accurate arm would throw out zero out of ten. Because he could only throw out those base runners that took 3.7 or worse to steal 2B. In college or pro baseball those that run 3.7 or worse don't steal bases.
Add the fact that the catcher with the wild arm is going to improve his accuracy the more he plays and it becomes a no brainer which catcher has a higher ceiling and a better prospect.
It is the difference of seeing the performance and scouting the player. Sometimes the guy that performs the worst is actually the better prospect. Maybe not the best player on that day, but the best prospect for the future.
My mother, if she were still alive, can tell us who the best player on the field today is. And sometimes that is also the best prospect and sometimes it isn't.
just to be clear... A strong accurate arm is better than a strong wild arm. But a strong wild arm is better than a weak accurate arm.