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To understand how to use the Related Product composite you need to know that everything else in an ONIX file is describing the very specific product as it is sold to a consumer. An ONIX file is like filling out another company's on-line product description for one ISBN. You want to know what to put in the Measure Composite? You pick up the book with that ISBN and use a ruler and scale and fill out the information. Can you measure a digital file? Then you don't use the Measure composite because you don't use metadata elements that can't apply to this product. (Wait: a PDF has dimensions built in to the file so maybe that's not true for all digital formats but there's never a need to say a product weighs or is sized as "0".) And so it for goes for every other composite: The Sales Rights composite describes the rights that the Publisher has on this product and not the rights they may have on the overall author's "work." The Supply Detail describes who retailers can order this product from and on what terms.
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Don't leave mad though, here's a thought game you can play: Can a collectible First Edition have a meaningful Related Product entry in the sense described here for other editions? 500 word essays will neither be accepted nor read, but BookNet Canada will collectively be amused and appreciate the effort.