https://www.gamespot.com/articles/e3-2018-fallout-76-everything-revealed-during-beth/1100-6459634/

Following a trailer and a few details during Microsoft's press conference, Bethesda has revealed even more about Fallout 76. During the Microsoft conference it was confirmed that the game is set in the hills of West Virginia, and you're one of the first to emerge into the Wasteland after the nuclear apocalypse. Furthermore, Fallout 76 is a prequel to the other games and four times the size of Fallout 4. However, appearing on stage at E3 2018, executive producer Todd Howard provided more information.

Howard said that Vault 76 was built to celebrate America's tercentenary, and you're selected to enter the vault and wait for the day when you can return to the world. During a gameplay demo, the player is given the command to leave by the Overseer, who says it's reclamation day.