Believe in me, I'm with the High Command
Can you hear me running? Can you hear me calling you? (Mike and the Mechanics)
Whatever derogation in local planetary sovereignty when a planet joins the Federation would have occurred. Probably some special status as a founding world. It may not be called "the High Command" anymore, but certainly some kind of local planetary government still exists. Although it is clearly specified that Sarek is an Ambassador, it is neither clear nor likely that he is either an Ambassador to Earth, or an Ambassador to the Federation (as today's Earthside nations send Ambassadors to the United Nations). My guess is that he is an "Ambassador at Large" representing the Federation; he must be referred to in Star Trek VI: The Undiscovered Country as "the Vulcan Ambassador" because founding worlds such as Vulcan and Earth get to appoint their own choices as diplomats externally representing a growing Federation. The name "Federation" pretty much dictates that there is a significant derogation in sovereignty of member worlds to the Federation, and thus there would would be no Ambassadors between Federation member worlds, or between Federation member worlds and the Federation. It's a federation, not a commonwealth, nor an intergovernmental organization where the member worlds still get to represent themselves externally with diplomats--that would imply a complete retention of sovereignty.
Member worlds thus particpate in federal government at the Federation Council (the UFP legislature), can seek election as Federation President or appointment to the executive (the UFP executive), or can become learned in the law and be either appointed or elected to the UFP judiciary ProfessorTrek 11:27, August 12, 2011 (UTC)