what document establishes the combatant command’s mission and areas of responsibility
Unified combatant command The Unified Command Plan (UCP) establishes the missions, command responsibilities, and geographic areas of responsibility of the unified combatant commands.[5] As of May 2018, there are ten unified combatant commands. Six have regional responsibilities, and four have functional responsibilities. Each time the Unified Command Plan is updated, the organization of the combatant commands is reviewed for military efficiency and efficacy, as well as alignment with national policy.
Joint Chiefs of Staff After the 1986 reorganization of the military undertaken by the Goldwater-Nichols Act, the Joint Chiefs of Staff does not have operational command of U.S. military forces. Responsibility for conducting military operations goes from the President to the Secretary of Defense directly to the commanders of the Unified Combatant Commands and thus bypasses the Joint Chiefs of Staff completely.