A Merchant ID (MID) is the unique identification number assigned to a merchant account, used by processors and card networks to route and settle that merchant’s card transactions. A business with several locations or sales channels typically holds a separate MID for each.
MIDs are the natural unit for volume-based programs: settlement happens per MID, so rewards accrual, program tiers, and reporting attach cleanly at the MID level. In Prestige Rewards each MID carries its own program tier, and a multi-location merchant sees earning per location roll up into one balance.