I have seen that sometimes stocks and broths can be used interchangeably in a recipe. I wonder - when can they NOT 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. 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.