Per the breed standard, the topline is level and the rump curves slightly to the set-on of the tail.
What two components make up the topline?
Neck, topline, body, and tail are allocated as 15 points in the standard. It's a big deal, and apparently one being neglected in both breeding programs and the show ring. Breeders first and judges second need to wake up and focus in this area.
Serious faults in the standard: roach-back, sway-back, and slab-sided.
With these being serious faults, they need to carry a heavy weight when evaluating your breeding stock. As many old time breeders would agree, it will take you four generations to breed out a bad topline, therefore, judges should think seriously before rewarding them in the ring.
There should be no dip at the withers or rise over the loin. This is what separates a Boston terrier from a Frenchie or a Bulldog. One should ALWAYS focus on length of loin. The longer the loin, the weaker the topline. The Boston terrier is to be a short bodied dog with a level topline, unlike that of a Frenchie/Bulldog.
Keep in mind, a great topline in a Boston would not be an ideal topline for a Frenchie or Bulldog.