In March 2022, the White House declared the ultimate goals of the DoD in their National Defense Strategy (NDS).
I did not read it, because as far as I know it is still classified, but I found different sources mentioning that nuclear weapons could be used in "extreme circumstances". For exemple, in a short commentary from "Arms Control Today" (ACT):
Biden’s policy declares that the “fundamental role” of the U.S. nuclear arsenal is to deter a nuclear attack, but will still leave open the option that nuclear weapons could be used in “extreme circumstances to defend the vital interests of the United States or its allies and partners,” officials told ACT.
Clearly, the possible use of nuclear weapon has to be declared, to work as the intendent deterrent (curiously, this means negating that MAD is the only scope of nuclear weapons), but what are exactly the vital interests of the United States? Are they clearly defined in geographic or economic terms?
For example, is it foreseeable that the Diego Garcia islands & other islands in the pacific/indian area will become vital interests in the medium-term (today they are simply interests)?