The Fleet Admiral of the United Federation of Planets, also called The Fleet Admiral of the Federation, The Fleet Admiral of the UFP or just The Fleet Admiral could be called either a title or rank of the highest ranking Starfleet officer that was granted by the Federation Council. It was considered be one rank above Fleet Admiral and was given as the ultimate honor from the Federation. Only one individual could hold the position and it was a lifetime appointment.
The rank of The Fleet Admiral of the United Federation of Planets had an equivalent in history with the rank of General of the Armies of the United States.
The proper way to address the individual holding this position was "My Fleet Admiral" (similar to calling a British Lord - "My Lord") and as "The Fleet Admiral" in writing or in referring to him/her. This was attributed to J.C. Montoya-Smyth, considered the second The Fleet Admiral, who was a British Lord and was referred to as My Lord Fleet Admiral, with the Lord being dropped near his death.
Like other Fleet Admirals, The Fleet Admiral was on active duty attached to no duty station and never retired. He could be placed on duty if Starfleet needs his services and has an office in the Needle which is located in San Francisco.(Star Trek: Black Hawk: "Altered Consequences")
The rank insignia was up to the individual when they wore their Starfleet uniform. Richard J. April wore a standard Flag Officer's uniform with a six pips within an admiral's bar. The previous two officers bestowed with the rank wore a flag officer's uniform wore the standard rank of Fleet Admiral of their time period.