Google Merchant Center has introduced a new feature called Custom Feed Matching.
This feature allows users to join a supplemental data source to their primary data source using attributes other than the ID attribute, such as brand or product type. This can be useful for adding additional product attributes based on these identifiers, like changing custom labels based on the brand field.
Similar results can be achieved with feed rules in the Classic Merchant Center, but these are harder to maintain than a simple Google Sheets supplemental feed. The feature is planned for release in the Classic Merchant Center and the new Merchant Center Next experience, called "custom data source matching."
For more details, refer to the Google help pages for Classic GMC and GMC Next.