The alternate version of Christine also mentions that Doctor Strange is from Earth-616. The number is significant because it's considered to be part of the "prime universe" in the Marvel comics, where a majority of the storylines take place. According to Screen Rant, Marvel Studios producer Nate Moore once said that the MCU is set on Earth-616.
While Mysterio namechecks Earth-616 in "Spider-Man: Far From Home" and also claims he's from Earth-833, this was likely just a fun reference from Marvel, considering Mysterio turns out to be a fraud at the end of the film. This makes "Multiverse of Madness" the first film that officially confirms the MCU takes place on Earth-616.
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