Unofficial hearing in the context of U.S. Congress is a hearing conducted by either single Congressmen of the United States or other state or local legislative bodies in order to hear the testimony of the people. It is unofficial because they are not conducted by either Congressional Committees of the United States Congress or by state or local executive or legislative bodies. They have no power to issue subpoena

Subpoena

A subpoena is a writ by a government agency, most often a court, that has authority to compel testimony by a witness or production of evidence under a penalty for failure. There are two common types of subpoena:...

s or enforce them. Attendance to such a hearing is entirely voluntary. Usually, they are conducted by members of the minority party.