Bethesda has explained in detail the new mode PVP that will soon arrive on Fallout 76Called Survival Mode. Regular experience will instead be defined as Adventure Mode and both will be accessible from the main menu.
In Survival mode, all players, with the exception of your team or group mates, are automatically marked hostile to each other by default. There will be no "invitations" to PvP combat and you can attack other players without restrictions from the first moment.
In addition, scaling levels in Survival mode will work the same way they do today in Adventure mode. As stated by Bethesda, this will help to level the battlefield a little and give the new characters a chance to fight against the high-level players they might encounter during their adventures in the Wasteland.
Bethesda plans to experiment with different rules after the beta, which should arrive by March 2019.