Assets are related to each through offers on the DEX.
If you make two standing offers to openly trade a pair of assets in both directions to set the trading value between them, then that relationship will stand so long as those offers exist. Others might make different offers at wildly varying relationships, which won't change the presence or value set on your offers. If many people agree to add offers near your valuation; or no one else makes any other offers but yours (like if you're the original issuer of the asset(s) then you have an effectively infinite supply to sell and others could only sell at a loss because your offer never changes so interest in creating a different rate wanes); then you have what you're describing.