Also called BN1, right?
Please explain me what is booster and what will it be used for?
Also feel free to give me additional insights other than included in my question about BN1.
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The Super Heavy booster is the first stage of the two-stage SpaceX Starship launch system. As Polygnome notes, BN1 is the name of the first prototype model of the booster.
The Starship upper stage, which has been making low-altitude test flights in early 2021, is not itself a useful orbital launch system; it may be able to reach orbit from Earth's surface, but not with any payload.