In section 4 "relevant mutual" means a body in respect of which the Secretary of State considers that conditions A to C are (and will continue to be) met.
Condition A is that the purpose (or main purpose) for which the body exists is to act for the public benefit by promoting the use by the public of services provided by a Post Office company or at its post offices.
Condition B is that the members of the body are—
persons who have an interest in the use by the public of such services (including employees of the Post Office company and other persons engaged in the provision of its post offices), or
persons who act on behalf of, or represent, persons within paragraph (a).
Condition C is that the body has in place arrangements for the prevention of disposals of property or rights by the Post Office company that would be inconsistent with the purpose (or main purpose) for which the body exists.