The useful picture here is the ending. Take 'peer', a British noble with a title, and wrap -age around it. That ending can close around a word in more than one way: it can gather many things into one body, and it can turn a person into the standing that person holds. So one word ends up covering everything a title touches — all the title-holders together, the single title itself, and even the printed book that lists them. If you are unsure which one a sentence means, look at the little word in front: 'the' points at the whole group, 'a' points at one title or one book.