Unified Command Plan (UCP)

The UCP is an Executive branch document prepared by the Chairman of the Joint Chiefs of Staff (CJCS) and reviewed and updated at a minimum every two years. It is classified as For Official Use Only. The UCP assigns missions; planning, training, and operational responsibilities; and geographic areas of responsibilities (AOR) to Combatant Commands.
The UCP states that communications between the President or Secretary of Defense and the Combatant Commanders shall be transmitted through the CJCS, unless otherwise directed. When significant operations overlap AORs, the UCP directs the Secretary of Defense to form a task force, determine its commander and assign it to the appropriate Combatant Commander.
For analysis of the UCP, see:

Feickert, Andrew, “The Unified Command Plan and Combatant Commands: Background and Issues for Congress,” CRS Report No. R42077. Washington, DC: Congressional Research Service, 2013.

Lucas, Nathan J. Defense Primer: Commanding U.S. Military Operations. CRS In Focus No. IF 10542. Washington, DC: Congressional Research Service, 2022.

For a map depicting the areas of responsibility of the geographic combatant commanders, see below:
UCP Map.jpg