For a Parent ASIN (Product Group), the best variation rankings within Parent will be tracked. Generally, it should be enough to only track keywords for one child ASIN within a parent, as this will also provide the rankings of the best-ranked child ASIN within the parent group.
For most sellers, only the best ranking variation ranking will matter, as it means the product as a whole is being found via the respective keywords.
It should also be noted that parents aren't actually real products, but merely artificial groupings of variations, so they themselves do not actually have any rankings.
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