There were many kinds of classification that could be used to distinguish Zergs and Predators, such as their size, their habitat, their diet, and so on and so forth.
But, the most common classification used in military operations was based on their lethality level and the numbers of troops needed to eliminate them. They were separated into, from the lowest to the highest, F, E, D, C, B, A, S, SS, and SSS.
F-Rank: the monster could easily kill a civilian, but any soldier with an exosuit and full arms could easily dispose of it.
E-Rank: posed a lethal threat to an individual soldier, but was easily handled by 5-man teams.
D-Rank: could destroy armored vehicles and would need at least a 10-man squadron to fight it.
C-Rank: Ordinary guns and artillery posed a little threat to the monster in this rank. At least a 50-man platoon and a mecha need to be deployed in the case of an encounter.
