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Endurance Testing and Production Driving: The Tesla Semi project involves two distinct groups within the company - the Engineering Group, which is responsible for endurance testing, and the Production Group, which handles the transportation of battery packs, drive motors, and Powerwalls between Giga Nevada and the Fremont factory. It's possible that some of the missing Semis have been deployed for these purposes.
Customer Deliveries: Another possibility is that some of the Tesla Semis have been delivered to customers for testing or permanent use. The head of the Tesla Semi engineering division, Dan Priestly, has mentioned that another customer would receive Tesla Semis later this year, in addition to the previously announced Pepsi order.