c++ array question please explain

I came accross this question in a book i recently bought and i am thouroughly confused.

-The number inside the brackets of an array definition is the ________, but the number inside an arrays brackets in an assignment statement, or any other statement that works with the contents of the array is the _________.

I have no idea, any clarification would be nice - thanks

The first blank asks about this number
char foo[BUFSIZ];

The second blank asks about numbers of this kind
foo[x] = y;

Chances are there are words you learned in class to talk about these numbers. If you can't think of anything then just describe the obvious.