As it relates to standards and certification, what is the difference between a brand and a retailer?

If the organization controls the design, development, and purchase of final products for sale under their own name or private label, they would be considered a brand in this case and require certification. You can find the full brand and retailer definitions, as well as many others in Appendix A in the CCS 3.0: https://textileexchange.org/documents/content-claim-standard-ccs/