This is there flank steak comes from. It is the abdominal muscles of the cow, on the underside of the beast, in front of the round and behind the plate. It does a lot of work, and has a really pronounced grain structure.
Marinate it, grill it hot and fast to medium rare, rest it, and slice very thin across the grain. (You aren’t doing anything chemically to remove the connective tissue that makes meat tough – so you have to do it mechanically.)
The rest of the flank generally find its way into ground beef.
Now let's move on to the round.