https://www.whitehouse.gov/presidential-actions/text-space-policy-directive-4-establishment-united-states-space-force/

(a) The term “United States Space Force” refers to a new branch of the United States Armed Forces to be initially placed by statute within the Department of the Air Force.

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Sec. 7. Associated Elements.

(a) A Unified Combatant Command for space, to be known as the United States Space Command, will be established consistent with law, as directed on December 18, 2018.

NB: The White House terms this a “new branch,” yet under the extant branch of the U.S. Air Force (“initially”), and to create a Unified Command and branch essentially performing overlapping responsibilities of the “new branch.” Also, the creation of a “new branch” requires Congressional approval (authorization) and resourcing (appropriation).

Branches of the Armed Forces don’t exist under currently existing branches; under this definition Space Force would be a Major Command (Majcom) of the U.S. Air Force
A major command is a significant U.S. Air Force organization subordinate to Headquarters, US Air Force. Major commands have a headquarters staff and subordinate organizations, typically formed in numbered air forces, centers, wings, and groups.Historically, a MAJCOM is the highest level of command, only below Headquarters Air Force (HAF), and directly above Numbered Air Forces (NAFs).

Air Force Instruction 38-101, AIR FORCE ORGANIZATION, 4 APRIL 2006 (with Change 2, dated 20 July 2006), paragraph 2.2.2., page 10.

A unified combatant command (UCC) is a Department of Defense Command that is composed of forces from at least two Military Departments and has a broad and continuing mission.

Title 10, U.S.C. Sections 161–168.

And see Joint Pub 1, p. GL-11.