Who are the Four Tier 1 Units in the U.S Military and What do the term Tier 1 Special Forces mean?

I might be wrong, but from what I understand only Tier One units are entrusted with executing the most delicate missions. Then come the Tier 2 units, which are also very highly capable, but their task is to provide support for the Tier 1 units. Tier 3 is the special operations units within the special operations commands of the different services. When a Tier 3 unit is attached to JSOC to provide support for a mission (for example if Pandur APCs from the 75th Rangers or helicopters from the 160th SOAR are needed) then for the duration of the mission this unit is reclassified from Tier 3 to Tier 2.

Tier 1 Special Mission Units are closed teams that are staffed by invitation. They fall under the ultra-secret Joint Special Operations Command (JSOC) which is within USSOCOM. They are the best of the best of the best. They frequently staff teams coordinated and commanded by the CIA’s Special Activities Division in teams called Special Operations Groups when full political deniability is required (clandestine missions).
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Видео Who are the Four Tier 1 Units in the U.S Military and What do the term Tier 1 Special Forces mean? канала USA Military Power