Product Relations were invented by a programmer that needed a place to store some data unique to a specific merchant. It is incredibly flexible but not pretty. Because of its flexibility it rapidly became a catch-all solution whenever we got a new request for something and it stuck around because it is the easiest place for us to hang new requests.
Video? Sure, put it in Relations!
User Manual? Sure! Put it in Relations.
How many product pictures do you want?! Sure, put it in Relations.
This feature varies so much between merchants, product types, and templates that I don't even know where to begin to document it that will make sense to everyone.
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