Feed rules
Transformations applied inside Merchant Center — find and replace, set a default, extract a value. Good for consistent, mechanical changes across the catalogue.
Supplemental feeds
A second file that overrides or adds fields per product. Better when the change is specific rather than rule-based: rewritten titles, custom labels, patched identifiers, per-market overrides.
Which to reach for
Rules for "add the brand to every title". Supplemental for "these 200 titles are rewritten individually". Most accounts end up using both, and neither requires touching the store export.