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in mmorpg you go out adventuring with many people at a time (sometimes up to 50 or more) and all people can be seen on screen at that same time
Let's clear this up. In a MMORPG, you are in game with all other game participants. There is no going out to adventure with parties. You are ALL in the SAME world, essentially. There could be hundreds, thousands, even a hundred thousand other players on that server with you, all interacting with each other. There is no need to go out in a party. You could, possibly, run into every single person in that world while adventuring.
That is the difference between PSO and a game such as Ultima Online. All characters exist in game at the same time while connected. You're all "in-game." Also, in a MMORPG, the game never technically stops. It continues without your existence. The game world, with all of its players, goes on indefinitely while the server runs and you can jump into that world and do whatever you want, play with whoever you want, die whenever you want... it's really pretty submersive.
Before I make this more confusing than it was before... yes, you do have "parties" but they aren't the people you're choosing to play the game with, really. You're all just decidedly together because you're teaming up, not because you want to get in the same game with each other. You're free to do whatever you want because those people exist with you whether you want them to or not as long as you play in the same world.
There's a HUGE difference between massive multi-player online and just multi-player online.