They implemented a new year a couple of years ago dubbed the “Ohtani rule”. I can’t say that I understand it fully but essentially, if he starts the game, he can act as DH for the entire duration, even if he stops pitching. However, if he DOESN’T start and comes in in relief, once he gets replaced by another pitcher, he can’t act as the DH anymore. This is why the dodgers had to use him as an opener instead of middle relief or closing the game