The comic book continuation of return as Smallville's Clark Kent and Lois Lane, respectively. Welling will be one of three actors to portray Superman in the crossover, with the other two being Tyler Hoechlin (Arrowverse) and Legends of Tomorrow's Brandon Routh (Superman Returns).
Superman prequel series Smallville starred Tom Welling as a young Clark Kent. The show centered on Clark's earliest adventures as he learned how to use his powers, forged relationships with various heroes and villains from DC Comics, and experienced some of the Smallville season 11.
Since Smallville Season 11 exists in canon, the events of the comic book series took place in continuity with Smallville, and may be carried over to the Arrowverse. If so, Smallville Season 11 could potentially provide clues to Tom Welling's role, and also to the main plot of the crossover. Smallville Season 11 dealt extensively with characters from alternate Earths and the Monitors. Both the Monitor and the Anti-Monitor are played by LaMonica Garrett, and will factor heavily into the story of "Crisis on Infinite Earths".
The Monitors were the main antagonists of Season 11. After a slow build-up, Clark finally had to confront them in the comic's final storyline. The main goal of the Monitors was to restore the natural order by erasing all of existence. After destroying Earth-2, the Monitors followed its only survivor, Chloe Sullivan, to Smallville's Earth, forcing Clark Kent to rally together nearly every hero he had ever teamed up with to take them on. With their combined forces, they succeeded in repelling the Monitors and saving the multiverse.
What this means is that Smallville's Superman has already gone through his own version of "Crisis on Infinite Earths", which can prepare him for what's to come in the Arrowverse's "Crisis". What Clark brings to "Crisis" is something that the other heroes lack: experience with fighting the Monitors. Clark saved his universe from the Monitors once before, and now he may have to do it again -- but this time with the help of Flash, Green Arrow, Supergirl, the Legends, and all the others.