The Semi-Tarrasch is a Tarrasch Defense with 3...Nf6 4.Nf3 thrown in. This changes the dynamics of the play dramatically. Black continues with ...c5 as before. The presence of the two Kingside Knights in the center means a different kind of middlegame from a Tarrasch.
You are specifically playing for an isolated d-pawn in the Tarrasch Defense. You're counting on this to give you activity. You are trying to avoid getting an isolated d-pawn in the Semi-Tarrasch. This is why ...Nf6 is played before ...c5. You will recapture with the Knight, not the e6-pawn if White takes on d5.
As soon as the Knight is in position, you are more than happy to play ...c5 and put the pressure on White's center. 5.cxd5 Nxd5 often follows and White can choose between the continuations, 6.e3, 6.e4 or 6.g3. Each have their own factors to consider.