These are used to control usage of corporate credit cards. MCCs are assigned by merchant type (e.g. one for hotels, one for office supply stores, etc.), with each merchant being assigned an MCC by the bank. Corporations can then control which MCCs their employees may use their corporate cards at, and this is enforced through the authorization system.
They are 4 digits in length. For example, MCC 5967 represents 'Inbound telemarketing merchants'.