A Perfect Level Up refers to a rare occurrence when a playable unit level ups and acquires all possible stat gains. This has an extremely low chance of happening due to the variations in the growth rates for each character. Pre-Promoted characters tend to have a lower chance than non-promoted units of gaining a Perfect Level Up naturally, due to their stats normally being set at lower rates as a result of their promoted status, whereas characters belonging to the Est archetype tend to have better chances due to possessing higher total growth rates than most other units. The only character to be guaranteed to obtain a perfect level up is Karel from The Binding Blade, due to his incredibly high growth rates, all of which are 100 or greater.
It is possible to use cheat codes to make a Perfect Level Up always occur. On the contrary, a Bogus Level Up (or Null Level Up) is when a unit levels up and gains no stats in the process. The only thing good about a Bogus Level Up is that it is still a step towards promotion. This can't happen in Radiant Dawn, as units will always gain at least one stat when leveling up, unless all their stats are maxed.