There are 5 Primary elemental types which are Earth, Water, Fire, Electric, and Psychic. Within each elemental type are sub-types.
Electric - Metal "tactical", Spark "tank", Magnetic "support"
Water - Ice "support", Liquid "tank", Vapor "tactical"
Fire - Plasma "support", Flame "tank", Molten "tactical"
Earth - Grass "support", Dirt "tank", Bug "tactical"
Psychic - Haunted "support", Dark "tank", Crystal "tactical"
Damage strengths and weaknesses are determined by primary types while the sub-types dictate the types of skills in each tree.
Each sub-type is categorized as a class, support, tank, or tactical. Each primary type has one of each class.
The support class contains skills that are based on survival. These skills tend to heal players, leech health, protect players, and increase defense stats. Monsters specializing in this class are really only useful in a party or group battling situation as they have very few damage dealing attacks. They are usually found at the back of the party protecting and healing the tank class monsters. Monsters with a high speed and high defense stat are the best suited for this class.
The tank class contains skills that are based on dealing damage. These skills are heavily focused on high damage dealing attacks that tend to take multiple turns, or have cooldowns. The tank skill trees have few if none stat affecting attacks, or healing skills. Monsters specializing in this class are useful in almost any scenario where a lot of damage need only be dealt once or twice. The only downfall is that these monsters tend to be somewhat inaccurate with their attacks, so they are highly strengthened with the help of a tactical class. Monsters with a high strength, high defense, and high endurance stat are best suited for this class.
The tactical class contains skills that are based on quick thinking. These skills tend to change the stats of friendly or enemy units, affect the status of monsters, and deal small amounts of damage while also causing status effects such as burn, poison, paralysis, etc. Monsters specializing in this class are fairly balanced in battle as they can deal fair amounts of damage while also protecting themselves. They are extremely helpful to have in a party setting as they can increase the strength of tanks, and the speed at which support monsters heal. A support class monster is very useful in combination with a tactical monster as it allows the tactical monster to focus it's energy on stat manipulation rather than self defence.
Most monsters species are only available with two of the three classes. In order to obtain a monster of that species with all three class trees, the player must specialize one monster in each of the available trees. The player then fuses these essences to form a third essence that will have all three trees available.
Some rare monster species will have all three trees available to all monsters.
It is also possible to find a common monster with all three trees available. These are extremely rare and are considered "glowing" for their unique and unusual coloring. Likewise, it is also possible to find a monster that is supposed to have two trees that only has one. These monsters are considered "corrupt" and have abnormally high stats to compensate for their lack of skills available.