The directions on some packages of raw food (frozen vegetables, dry beans, dry pasta, rice, and the like) indicate that one should first boil water, then add the food, wait for the mixture to boil again, turn the heat down, and cook the stuff for some length of time. The directions on other packages indicate that one should combine the water with the food first, bring it to a boil (or to the boil if you're British), turn the heat down, and cook the stuff for a length of time. (That is, the latter directions don't include boiling the water first: the water is heated with the food in it.)
What determines which types of food are cooked each way? Or is one set of directions right (for all foods) and the other wrong?