I'm just going to start with PCL pitching prospects as an example. They are close to big league ready and shouldn't develop a lot from here, so we can figure most of these players won't change levels a lot. The basis is simple. The first letter is innings, the second letter is K rate, the third BB rate.
Taijuan Walker heads the list. He is a 20APA. The APA is usually a dominant reliever or a top of the rotation starter without a lot of innings. Well, Taijuan will be a starter and will log more innings than this, so you can move him up to a PPA. These are aces. That's Walker's potential. If his K rate drops, to a PAA, he's still a #2/3 starter. If his command regresses to a PAB, worst case, he could be Daniel Cabrera like. To me, that's the absolute floor. I don't really see a case where Walker is healthy and he isn't effective.
Jarred Cosart is another interesting case. As a 23AAB, he fits along with a lot of guys who show promise but don't have the command to be high level players. One of two things will likely happen. His command will improve making him an AAA, solid pitcher, then allowing him to throw more innings, a PAA, a #2/3 starter..or he will get moved to the pen, where his stuff will play up and his K rate will go up to an APB or even a BPB. Either way, that's a high end reliever. If he lacks K's in the pen, he could be a BAB, a middle reliever. That's his floor.
A couple things that seem intuitive to me but may not be when using this system. If you move a player up in innings, lower him in K rate.
Lower a player in innings, may be able to increase K rate. The higher the GAPPER score, the more likely that will work, generally.
Command increases are hard. Do not assume it will happen but generally, the younger the player, the more likely he will improve.
The Plus K rate is isolated to 7.5% of all MLB pitchers. 3.17% fall into the Below average innings pitched range, 3.63% is in the average innings pitched range and only 0.69% of players with a Plus K rate are Plus innings pitchers. It is very rare.
Let's look at a couple in the midwest league. Lance McCullers is an 19APB. That's a big armed reliever. That may be what he is but lets try this..Lets bump his innings while maintaining his K rate, PPB. That's Nolan Ryan, Randy Johnson territory, so lets move him to a PAB. Again very rare, filled with 100+ BB guys, so the likelihood of him being a big league starter is very low without command improvements.
Let's reset. Start back at APB. If he'd improve command, he could be a APA, dominant reliever or high end starter with limited innings..that makes me think we should put in the rotation full time and lead him to a PPA. Well that's an Ace. He does have ace potential but if his K rate drops with more innings hes a PAA or a #2 or 3 starter. That seems more likely.
In the end, a very wide array of outcomes exist with McCullers but considering that he has a high GAPPER score, the odds are fairly good that he improves.
The same things could be said for Daniel Norris a 20APB. The difference is Norris' GAPPER score is much less but still well above 0, so there is a good chance he improves as well, just less likely than McCullers.
I hope this explains a little more. If not, hit me up @mattgarrioch on twitter.