High Commissioner was a position within the bureaucratic structure of the United Federation of Planets. These officials have expansive powers under the Galactic Emergency Procedures, including the exercise of authority over Starfleet officers. In addition to overseeing relief missions, commissioners also perform diplomatic duties.
In 2267, on stardate 2823.8, Galactic High Commissioner Ferris, acting under Title 15 of Galactic Emergency Procedures, ordered Captain Kirk to abandon the search for the shuttlecraft Galileo and proceed immediately to Makus III to deliver urgently needed medical supplies. (TOS: "The Galileo Seven")
Federation Assistant Commissioner Nancy Hedford had broad powers as well. In 2267, she was dispatched to Epsilon Canaris III to prevent the outbreak of war. She, likewise, continually issued orders to Captain Kirk. Unfortunately, Kirk was unable to comply as their shuttlecraft had been commandeered and rendered powerless by the Companion. (TOS: "Metamorphosis")
Officials of this type were less frequently seen in the 24th century, possibly due to starship captains (especially captains of Explorer type ships such as the USS Enterprise (NCC-1701-D)) having received more diplomatic training and competency in general.