The title for these positions was renamed to remove the word "Principal" as part of the National Defense Authorization Act for Fiscal Year 2018.
The use of these titles was severely restricted by the FY2010 and FY2011 National Defense Authorization Act (NDAA).
[8] Initially, differentiation between the PDUSD and DUSD was not clear, with some positions officially referred to by both titles.
[11] Four of the five Under Secretaries of Defense had a PDUSD reporting to them, of which two were designated by Congress and appointed by the president.
In early 2009, concerned about the proliferation of the DUSD title and the lack of clarity of its role in the DOD organization, Senator Carl Levin, then the Chair of the Senate Armed Services Committee sent a letter to the Deputy Secretary of Defense requesting clarification on the role of the DUSD.
Two positions were eliminated by restructuring responsibilities, and four of the DUSDs were given new titles (Director, Deputy Comptroller, and Chief of Staff).
[15] The remaining five positions were considered by the DOD to be responsible for "major pillars of activity" that required a sufficient title to indicate their importance, but "do not warrant being elevated to the level of a PAS official".
[19] This title was chosen as it "properly convey appropriate status and stature for these principal policy advisors to the" Under Secretary of Defense for Intelligence.