POTENTIAL.
In this fic, a Pokémon's potential, so far, is defined by Elemental Property-types.
The potential they have, sets the Stage/Rank of strength Pokémon can—safely (health, basic requirements and with training)—100% reach.
Now, just because it's standardized doesn't mean that's their absolute limit. Pokémon with only one type have a chance to push past their natural potential "cap". If a Pokémon has more than one type and both are at the same potential level, they'll stack and have a slightly better shot at breaking their "cap."
Example: A single-type Pokémon with Pseudo-Elite Four potential has a 50% chance of breaking through its limit. If a Pokémon has two types both at Pseudo-Elite Four level, it would have a 60% (30+30) chance of breaking through.
How the Pokémon World and the MC Check Potential
The League and other organizations check a Pokémon's potential through their genetic data. For example, when a trainer get a checkup at a Pokémon Center, the Pokémon genetic data runs through an algorithm—one only they know about—to determine a Pokémon's potential.
It's important to know that this doesn't work for every Pokémon across all regions. A huge amount of data is needed from the same Pokémon species to standardize this info. Up until now (Ch. 80), it only works with Pokémon from Kanto and Johto. Maybe it would take genetic data from over 10,000 Charmanders just to properly standardize that single Pokémon species.
Add to that the fact that even 20 years into the future, trainers still have to catch Pokémon in a Poké Ball first before they can even start analyzing the potential data.
This tells us that the world isn't built on a fixed potential system since, just like anything in life, potential can be improved—though it's definitely not easy (Believe in yourself, little one, you can do it!!)
How the MC Check Potential
The MC's ability to check potential isn't OP either. It doesn't work on every Pokémon and is actually limited by types.
The MC's "special power" is based on Dark-type Energy, meaning he can only check the potential of Pokémon that have an advantage or disadvantage against Dark-types—and even then, it's just a general sense, he used years of hard work just to achieve this in his past life.
The MC does this by "detecting" the intensity of emotional feedback, usually "Contempt" for Ghost and Psychic types, and "Disgust" for Fighting, Bug, and Fairy types.