"Evolution" is the greatest feature of a Digimon! Their "Level" increases each time they evolve, and their appearance changes drastically, increasing their power, gya. This time, I'll be going over those Levels, gya.
The "Eggs" of Digimon. Their patterns differ based on the species of Digimon that hatch from them and the region that they are from. When a Digimon that is powerful enough dies, it will leave behind a Digitama.
A state in which their combat instincts have awakened as their intelligence has developed and they have started to get stronger. Although they can fight, they are still growing, and their subsequent evolution will change drastically depending on how they grow. (This illustration is of Agumon)
A state in which they have fully grown as a Digimon, becoming even more powerful and intelligent. Most Digimon stop growing at this Level. (This illustration is of Greymon)
A form attained by exceptionally powerful Digimon among Adults. They possess overwhelming power compared to Adults, and when living in the wild, they often become the leader of the pack. (This illustration is of Metal Greymon)
A form attained by Perfects that have broken through their limits by surviving many battles. They are Digimon that often act alone due to their incredible power. On rare occasions, there are individuals that possess power surpassing the Ultimate Level. (This illustration is of War Greymon)
A phenomenon in which the structural information of their body changes drastically as a result of taking in large amounts of other data, increasing their power. In order to survive, they evolve into various forms, even those of same species, according to their habitat and latent abilities.
Agumon Hakase's Review
There are other Levels such as "Armor" and "Hybrid", but this time I've summarized the basic ones, gya. Only a handful of Digimon are capable of reaching the immensely powerful Ultimate Level. The reason why Digimon evolution doesn't have a straight path is because it is a means of survival in a world where the weak are food for the strong, gya.