The JAL has made a late (and very minor) addition to the rulebook with 10 days left til the season. The new rule reads that "Once a pitcher is replaced they cannot pitch again in the game." Previously there was no rule saying that a team could do re-entries at pitcher, but there wasn't a rule against it either. As a result, from time to time teams have indeed subbed out a pitcher and then subbed them back in later in the same game.
League administrators say that re-entries at pitcher were never intended to be part of the game, but just sort of happened over time since there was no clarification. They add that luckily no team had ever blatantly abused the "non-rule" but could have seen it become an issue in the future with lots of teams having good relief pitchers and dual aces now. In essence, teams could literally have had their pitchers switch off every batter to stay fresh. JAL officials say that would have been "very silly and probably controversial."
The newly added rule will basically make pitching sub procedures like normal baseball. In the MLB for example, if a pitcher is subbed out of the game they are done and cannot re-enter anywhere. On rare occasion in baseball, teams will do what's called a "double switch" in which the pitcher is subbed out for another, but simultaneously subs out a player at another position (such as left field) so that they can sub back in at pitcher later. In the JAL this will not be the case. When you're done at pitcher you're just done at pitcher.
With this rule remember that a subbed out pitcher can still go play the field and that fielders are still allowed to sub in and out all game long. This new rule also obviously does not effect batting because the JAL is not like baseball in that players aren't required to play both sides of the ball. In case you're also wondering about batting, there is (and has been) a pinch hitting rule in JAL that says if a pinch hitter is subbed in for a batter, then the batter subbed out cannot bat again.