Destiny’s next expansion, Rise of Iron, launches in September, and we know that it’ll include a new raid, Strikes, weapons, gear, and playable area. But Bungie’s decision to leave behind PS3 and Xbox 360 for this expansion has caused some confusion among players. Recently, a couple members from Bungie discussed Rise of Iron and clarified a little about why it’s only coming to Xbox One and PS4and how the playable area will interact with the existing world.

In an interview with Kotaku, creative lead Christopher Barrett and marketing director Eric Osborne shared some new details about what fans can expect from the expansion. Specifically, they stated that the decision to only develop for current-gen platforms allows the studio to add more content than would otherwise be possible. “In order to add more stuff, we would have to remove stuff from the game to still support legacy content,” Barrett said. “So it’s really about the amount of stuff in the game. We’re always looking to add new features and new graphical improvements.”

He continued: “Anything that’s additive at this point, something has to go. Big stuff. So [leaving 360 and PS3 behind] allows us to make the bucket bigger. And there are new bells and whistles and fun features we can do… To be clear we’re not turning any of that old stuff off–you can still play Destiny if you’re sitting on your 360 and you’ve just picked up The Taken King.”

The developers also explained more about how the Plaguelands will work. The Plaguelands is the new playable area coming in Rise of Iron, and it’s set on Earth. Since this is the first time in Destiny that two different playable areas will be located on the same planet, there’s some uncertainty about how the Plaguelands and the other Earth zone, the Cosmodrome, will interact. Barrett and Osborne clarified that the two will be separate, but the events of Rise of Iron will affect the look of the Cosmodrome.

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“It’s separate,” Osborne stated. “That allows us to do really cool things like where you’re first resurrected [at the beginning of the game] and you have, you know, the line of cars, that’s completely blanketed in snow and the water’s frozen, and there’s a giant Fallen ketch back there, and you sort of punch through the wall… It’s the first time we’ve really done that, that permanent time movement for [players starting a new character].”

Destiny: Rise of Iron will be released on September 20 for PS4 and Xbox One. At E3 2016, we sat down with Barrett as well and talked about the reasons for shifting exclusively to current-gen consoles. We also talked with him about the expansion’s raid. Recently, Bungie also announced that the expansion won’t have a collector’s edition; a $30 download is the only way you can purchase it.