It's giving you permission to pirate Halo: The Master Chief Collection to play Halo: Reach.
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thank you! Now I get it
You can buy any one game separately. The master chief collection is just a bundle of all the halo games. So if you just want to play halo reach just buy halo reach.
Just FYI you can't play any of them offline. So if that's your jam be aware of that limitation.
They set it up weirdly but it's because it has a launcher that you chose what to play iirc so in actually TMCC is a launcher that acts as a bundle that's a base game in steam while every game acts as a DLC for said main game, but when you buy the "DLC" you get the launcher with it.
it hurt itself in its confusion!