|The following is based on Fallout 4 cut content and has not been confirmed by canon sources.|
zUnusedTheater46A is an unused exterior cell in the game files of Fallout 4. It is marked as "Quick Test Cell" in the local map, a name shared by several other cells.
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The only way to exit the cell is to use a console command such as coc SanctuaryExt to teleport out. This is only possible on PC.
zUnusedTheater46A appears in the game files of Fallout 4 as a test cell.