Fire Caste are stronger, faster, and bulkier than the others, which is why they are the Empire's warrior caste. Earth Caste are shorter and stockier than the other castes, but they are more skilled in practical crafts, which makes them good engineers and scientists. Air Caste are tall, thin, and relatively brittle, but they have better reaction times and are better suited to low gravity environments, which is why they serve as pilots, naval personnel, and messengers. Finally, the Water Caste are somewhere in the middle of all of these, but they are more adept at learning alien languages and techniques of communication, making them excellent traders and diplomats. TL;DR Fire Caste is soldiers, Earth Caste is builders and scientists, Air Caste is pilots, and Water Caste are diplomats and traders.
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