A merchant ID number is a 15 digit unique identification number assigned to the business entity when they open a merchant account. This number is used to determine a merchant who is tackling to process the transaction to both the processing company and the customer’s bank. This unique identifier is the reason sales are made by a given merchant. This identifier may eventually lead to money being transferred from the cardholder’s bank to the merchant’s bank account. Without a merchant ID number, the card-issuing bank wouldn’t be able to know where to send the money.