Oh, perhaps I have the name wrong. I meant the part connected to the payload with the Merlin engine designed for vacuum, above the three main rockets (the center of which must be the core).

That's the 2nd stage. It's still attached to the payload I believe, so that they can test mid-course corrections to the transfer orbit.

It will relight its engine again in about five and a half hours to depart Earth orbit for interplanetary space.

Thanks.