Something that is described as specified is explicitly named or stated. If you assign seats in your classroom, you ask the students to sit in specified seats — according to your seating chart.
The word specified and several related words, including specify and specific, all have at their roots the Latin word species, meaning "a kind of" or "a sort of." All these words express the idea of naming things explicitly or categorizing them. If you're baking, you should try to use the specified ingredients and amounts or things won't taste right.