Like I said, it's probably the hidden value of a Pokemon, or trait value if there is indeed such a thing. Hidden Values have no effect on a Pokemon really, but they exist. In RSE Hidden Values are also what decides if Mirage Island will show up or not on certain days. Hidden Values I THINK are also what determines the gender of a Pokemon. Genderless Pokemon I'm pretty sure have a hidden value of 0.
I remember reading that Gender Value is a number between 0-256. If the value falls between 1-128, that pokemon is a male. If the value falls between 129-256 it is a female. If the number is Zero, that is a mirage Island pokemon. Even genderless pokemon have a Value, so they dont assure sight of Mirage Island. This info comes from serebii