42 CFR §421.401

For purposes of this subpart— Appropriate MAC means a MAC that has a contract under section 1874A of the Act to perform a particular Medicare administrative function in relation to: Medicare Administrative Contractor (MAC) means an agency, organization, or other person with a contract under section 1874A of the Act.

(1) A particular individual entitled to benefits under Part A or enrolled under Part B, or both;

(2) A specific provider of services or supplier; or

(3) A class of providers of services or suppliers.