Every company that accepts credit and debit cards (both online and offline) is given a merchant account ID by their payment service provider once they have set up a merchant account. Depending on their business models, each merchant account has one or more merchant ID numbers.

Example 1:

Your online business only runs in the domestic market, you just need a merchant account with one merchant identification number.

Example 2:

You have multiple websites for different regional markets, each business channel would have a distinct merchant identification number (MID). However, they are all under the same merchant account ID (same merchant account) because they belong to one business.