What are the NIV cross-references? Where do they come from?

In my NIV Bible I see tiny letters inside the text and references to other passages in the margins. Some of these are tremendously helpful, some seem only to relate by the use of one word. Where do these cross-references come from, did they originate with the authors of the NIV or are they from some older source? On what basis did the creators of this system decide that passage A should be related to passage B? Do we know the intention of the authors for how they should be used?