Rhimes told TV Line that the episode, which imagines what life would be like if Meredith's mother Ellis had not passed away, is not imagined or dreamt by Meredith (Ellen Pompeo).
"It's not a journey that Meredith is aware of," she said.
"The episode starts out as sort of a 'What if?' question that's asked in the voiceover. We just really wanted a chance to explore the 'What if?' of it all."
Rhimes also revealed that the instalment will give viewers a better understanding of the doctors, saying: "There's an unarticulated longing in that episode for the characters. The sense that something's wrong and something's lost, but they don't quite know what it is because they're not themselves."
Rhimes has previously hinted that Meredith will be "bright" and "shiny" in the alternate world, while returning star Kate Walsh has compared the episode to the movie Sliding Doors.
Meanwhile, ABC's president Paul Lee has claimed that he is confident that the show will be renewed and Patrick Dempsey has admitted that he would consider returning for another season.
Grey's Anatomy airs on Thursdays at 9/8c on ABC.