When broths and stocks can't be used interchangeably?
From WIKI they seem to be the same thing.


  Stock versus broth
  
  Many cooks and food writers use the terms broth and stock
  interchangeably. In 1974, James Beard wrote emphatically that stock,
  broth, and bouillon "are all the same thing".
  
  While many draw a distinction between stock and broth, the details of
  the distinction often differ. One possibility is that stocks are made
  primarily from animal bones, as opposed to meat, and therefore contain
  more gelatin, giving them a thicker texture.[3] Another distinction
  that is sometimes made is that stock is cooked longer than broth and
  therefore has a more intense flavor. A third possible distinction is
  that stock is left unseasoned for use in other recipes, while broth is
  salted and otherwise seasoned and can be eaten alone.
  
  In Britain, "broth" can refer to a soup which includes solid pieces of
  meat, fish, or vegetables, whereas "stock" would refer to the purely
  liquid base. Traditionally, according to this definition, broth
  contained some form of meat or fish; however, nowadays it is
  acceptable to refer to a strictly vegetable soup as a broth.


A possible difference a broth is a byproduct of cooking and you make a stock.