When the first cut is taken off the brisket, what remains is this thicker half. Because it is fattier, it also cooks up juicier and with more flavour.
The fat on it is called the “deckel” or “deckle.”
This is the better brisket cut to use because it has the layer of fat. Cook it fat side up, so that the fat will melt into the meat, keeping it moist.
The fat will melt off with slow cooking into the meat and gravy, and then you can just skim it from the gravy.