From Halopedia, the Halo wiki
Plasma jockey is a translated colloquial Forerunner term used by the Miner rate to refer to their stellar-class engineers. Equipped with third-class ancillas, plasma jockeys were tasked with "taming" young stars for future use. While most plasma jockeys succeeded in their tasks, many simply disappeared after several centuries, and no one knew what became of them.
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