Word for "something that is only useful if all needed parts are present"? [closed]

Solution 1:

If a part cannot work on its own it means it is not self-sufficient, not autonomous, not independent. If you look at the opposite of autonomous you get dependent or interdependent. You may also think of incomplete or complementary. But from your question it is not clear if an adjective would be ok as an answer (which you could add next to part or component)

I am afraid essential does not go here, because though it does not state that the component is self-sufficient, the emphasis falls on the fact that the component is significant for the whole, not on the fact that it cannot function without the other parts.