During the Developer Update Livestream for Season 2, Season of Blood, Blizzard announced an update to the content progression in Diablo 4 to add more variety on your journey to level 100 and introduce target farming Uniques to the game through five newly added endgame bosses.
Diablo 4 Season 2 – Season of Blood Developer Update Summary
The flow chart below shows the typical content progression for a player when Diablo 4 first launched.