Rank is a feature in Jade Cocoon 2. Firstly, it loosely shows the player how high any given Divine Beast's HP (Hit Points) and MP (Mana Points) are. Rank, in this sense, is seen in the Beast Notebook and can be seen on all entries, including Kalma.
More importatntly, Rank determines (by extension, slightly loosely) the power of Skills and Magic, damaging and otherwise. Generally, the higher the moves rank, the more powerful, useful or harmful it is. In addition, higher ranking moves generally cost more MP but can also have side effects, such as Kamikaze Hit and Limit Break.
Move rankings are also seen in Seed Beast entries, where ranking categorizes different ranking Seed Beasts (by move) separately, which also determines how much they cost to use in merging and how much Yan and Reputation is earned by the player upon defeating the Kalma infested Divine Beasts in the Wormhile Forest.
The ranks go from E (lowest) to A, followed by S (highest). Rank E is not used for HP and MP ranking.