Does Rebellion include Trinity, and therefore the original game?

While it is true Rebellion has all the previous content ever developed for Sins, unlike Trinity it is not backwards compatible. You can only play Rebellion, you do not have access to play trinity/diplomacy or the original game by itself.

That said, there are really only two cases where this is an issue. One is if your friend has one of the older versions and you'd like to play with them. The other is if there's a mod you want to try, though most of the mods are being updated for Rebellion. If neither applies to you I'd say you aren't missing out.

OK. I only ask since I am planning to buy this on Steam, but aren't sure if I should buy both Trinity and Rebellion or just Rebellion. Which gives me everything?

Rebellion.

OK. Now is it what I think it is, where there is a dialog box that allows me to select the game and expansions I wish to play?

No. That's what I've been trying to say. It is not backwards compatible. You can only play Rebellion, you can not play the previous versions without the Rebellion content. Trinity had that but Rebellion does not.

What exactly are you worried about that you'd want to play the old expansions?

What I am worried about is spending twice for the same stuff. I want to know what Rebellion has. If it says "all previous content", I assume that means Trinity. Trinity has the original game and the two expansions, Diplomacy and Retrenchment. If I get Rebellion, is that considered buying Trinity, and therefore Rebellion being a bundle in disguise?

If you buy Rebellion, you have every contents from the original game, Diplomacy and Entrenchement, plus new content (like subfactions, corvette, titans). So if you want everything, go ahead and buy just Rebellion. The only thing you can't do if you have Rebellion, is playing online with people who only have Trinity

OK, thank you. I know what to buy now.