I can't figure out what it is. There are two extra wide bays making up most of the building. This could be used for casting, however we don't see the same in Austin where there are standard width pillars.
There were also no particularly deep foundations (unless they were done after the walls went up), which could also mean casting but is not a definitive answer.
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However while there is some air handling equipment on top, there is far less than on the model Y casting building. There is also far less than I would expect for cell production.
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Model Y casting roof for reference.
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If I had to guess, I'd agree with
@Chenkers that it's the design studio - but only because it seems too different to the other building's we've seen go up for casting or other standard manufacturing buildings.