Assistant Federation Commissioner was a Federation title.
As of 2267, Nancy Hedford held this title. That year she was dispatched to Epsilon Canaris III with the goal to prevent the outbreak of war; her mission was cut short when she developed Sakuro's Disease and had to be recalled. (TOS: "Metamorphosis")
Hedford was twice called "Assistant Federation Commissioner" in formal cases, but was also addressed simply as "Commissioner" throughout the episode.