The number of digits in a merchant identification number, or MID, varies, but is commonly a multiple of 4, notes Merchant Partners. Common lengths include 8, 12, and 16 digits.
A merchant identification number is a unique identifier assigned to a merchant account to provide identification for processing activities such as transactions, adjustments, chargebacks and end-of-month fees, explains Cayan. There are three components to a merchant account: an MID; one or more terminal identification numbers, or TIDs; and a gateway identification number, or GID.
When the individual supplies his credit card information to an online shop, the payment gateway identified by a GID encrypts the information and sends it securely to the Internet merchant account, reports PayPal. The transaction then undergoes a review for authorization from the customer's bank, and the encrypted result travels back through the gateway.