Usually, it's the highest possible level for each species, each area, each encounter method, each time (when applicable), and each version. Sometimes, Pokemon of the same species but different levels will be registered separately. For example, let's imagine that, when you use a super rod at Petalburg City without using an ability, there's a 98% chance of encountering a level 20-35 Corphish and 2% chance of encountering a level 20-45 Corphish. When pressure, hustle, or vital spirit takes effect, the chance of a level 35 Corphish gets increased by 50% relative to the 98% or 49% relative to the entire sample space. Likewise, the chance of a level 45 Corphish gets increased by 50% relative to the 2% or 1% relative to the entire sample space. This results in a 49% chance of a level 35 Corphish, a 1% chance of a level 45 Corphish, and a 50% chance that the level is determined as normal (from my trying to get a level 45 Corphish, this seems about right).