NMDurham and Bung1eIsSoLame aren't wrong. Go to any digital store and look at the EXPANSION Pass for Destiny 2. Want research for their opinions? How about an official picture.
http://cdn3-www.playstationlifestyle.net/assets/uploads/2017/08/destiny-2-expansion-pass-555x328.jpg
Note... "Expansion Pass" in big bold letters for your viewing pleasure.
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2 or 3 dlc's in a single pass make an expansion like most games
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Curse of Osiris is Expansion 1 according to Bungo repeatedly throughout their website. https://www.bungie.net/en/pub/CurseOfOsiris Not Expansion 1 Curse of Osiris part 1 or anything like that. Just... Expansion 1. I will tell you though you are right for most games, I will note Bioshock Infinite's Burial At Sea content with this picture. http://www.digiseller.ru/preview/197847/p1_40902202742217.JPG So while NMDurham and Bung1eIsSoLame aren't wrong, neither are you. But to Bungie, this is how they are treating Expansions and DLC's.