Judging by the first episode, the usual level system doesn't even apply here, with Shoutmon, who would likely be a Child digimon in any of the other series, beating up Ultimate-level digimon like Mammothmon. So, one can logically infer from this that since digimon aren't bound by levels and strength, they don't evolve like they do in previous series, and are thus in a more nebulous state where any digimon can take on another, as long as they're strong/tough enough. Which brings us to...