So there's this position
and this position
and this position
And even though all of them are about black snatching the pawn from the Queen's Gambit, they all play completely differently. In the course I'm working through, they're in three different chapters. The first is called the Slav (or according to Chess.com, the "Slav Defense: Modern, Two Knights Attack"), the second is called the Semi-Slav (or according to chess.com, the "Semi-Slav Defense Accepted"), and the third, why, the third is the Queen's Gambit Declined: Modern Variation, 4...dxc4. Totally different.
Also in case you were wondering, this is not confusing at all.
At least now I know why even after reaching 111111111118888888888888888888888000000000000000000000 I could never figure out when you are and are not supposed to take-and-then-defend this pawn and what to do about it if your opponent tries. Turns out there's a huge and pretty crazy theory on how to play in all cases. Except he third one of course, since that just plays like delayed queen's gambit accepted. Obviously.