Since the beginning of Sentinels of the Multiverse, the official order of operations has always been to resolve actions in the order they came into play. There are a lot of reasons for this, but the primary reason was to keep the game moving smoothly. At the end of each villain turn, there shouldn't have to be a big, round-table discussion about each card in play that has "end of turn" text and what order they should go in. It's simple enough to keep the cards lined up in the order they came into play and to follow the text instructions in that order. Thus, when cards would activate "simultaneously", what determined the card order fell to what had been in play longest.
However, there are certain interactions in Sentinels of the Multiverse which are made significantly less smooth by forcing simultaneous actions to be determined by card play order, such as Legacy's cards "Superhuman Durability" and "Fortitude". Therefore, to clear up such interactions, there is a new ruling.
Any cards with "start of turn" or "end of turn" text happen in order of cards played, as before. However, anything else that acts "simultaneously" goes in whatever order the players choose, as with all ambiguous situations in Sentinels of the Multiverse.
This will affect certain interactions in the game, but all of them in ways which benefit the players. Enjoy your newfound control, heroes!