MID
Merchant IdentifierIdentifiers
The unique identifier assigned by the acquirer to a merchant for processing and settlement. Each MID corresponds to one combination of legal entity, MCC, and acquiring bank. Merchants with multiple business lines or regions often hold multiple MIDs.
In practice.
The MID is your actual merchant account identity: terms, reserves, descriptors and monitoring all attach to it. Serious operators often run several MIDs across acquirers for redundancy and routing, which is legitimate when each is properly underwritten. Spreading volume to hide a chargeback problem is not, and schemes look for that pattern.