Warning: SPOILERS for Crisis on Infinite Earths Part 4.
Crisis on Infinite Earths made the DC Extended Universe part of the Arrowverse's multiverse thanks to a surprise cameo by Ezra Miller's Flash - and even revealed how he gets his name. Since it began on December 10, viewers have been eager to find out how the massive 5-hour crossover would conclude, with Arrow and Legends of Tomorrow's episodes wrapping up the story. During the fourth hour of the event, Barry Allen (Grant Gustin) has one of the more intense experiences out of the Paragons. During a complicated trip through the Speed Force, Barry suddenly finds himself in STAR Labs talking with a doppelgänger version of himself. Despite the Multiverse no longer existing, Barry is somehow talking to another Barry from an unknown universe.
After years of fans and even Miller himself hoping to see the Flash of two worlds come together, the crossover had the DC TV and cinematic universes cross paths. While the encounter is brief, it was a major moment for Warner Bros.' DC properties as this was the second time TV and film versions of a character came face to face in this crossover. The first one happened with Tyler Hoechlin and Brandon Routh's respective versions of Superman during the first half of the Crisis.
The two Flashes compliment each other's costumes, as one prefers the armor while the other enjoys something softer. However, it's never explained how Miller's Barry got to this point. As the two Scarlet Speedsters engaged with one another, Gustin's incarnation accidentally gifts Miller's version with an important part of his identity. As the DCEU Flash tries to figure out who this stranger is, the Arrowverse version reveals his name to be The Flash.
The other Barry's confusion over the name confirms that he has never been called by that name in his world. During the events of 2017's Justice League, the fastest man alive never received his proper superhero name at any point, since he was still a hero in training. Even after the League's first mission and becoming a united front to the world, Barry was still not going by The Flash. While it's hard to determine how much time has passed between Justice League and Miller's appearance in Crisis on Infinite Earths, this scene means that Arrowverse-Barry basically got to name his movie counterpart.
With The Flash movie set to hit theaters in 2022, Miller's guest spot could (in theory) be referenced with regards where he gets the name. It would also establish the Multiverse's existence at the get-go in the new franchise. But for viewers who had been wanting to see Miller back in the suit sooner rather than later, they got it thanks to The CW - with the added bonus of seeing Miller share a scene with Gustin. Despite the fact that the ending of Crisis on Infinite Earths never clarifies the status of the DCEU in the new Multiverse, it wouldn't surprise anyone if Gustin and Miller one day share the screen again.