What is the point of service chiefs in a case where combined arms military regions exist? There exists positions like commander or chief of staff of army, navy and air force in militaries that have regional commands that include all units from all services within their area. What do service chiefs do if all the units in their service belong to regional commands? If a fighter squadron is under a theater command from the army, what is the airforce unit's relationship with their service chief? Who gives them orders? Can both the theater command and service command give orders? What if those are not compatible with each other? Are service chiefs even needed when all the units are under theater command?

Is there known differences in how other major powers than US, like China and Russia do this?