All sub-catalogs are part of the root catalog. You'll need a separate sub catalog if you want promo codes that only apply to that group of courses, because that promo code applies to all sub catalogs underneath the catalog that it's associated with, as far as I know.
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Echoing what mzimmerman said, each subcatalog would be associated to the root catalog. If any promo codes are created at the root catalog level, then each subcatalog would be able to see those. For promo codes to be showing for courses in a specific subcatalog, we would need to enter that subcatalog when creating the promotion. That would be the Choose Catalog field in this guide: