Acting or Under Secretary of State   Cordell Hull was Secretary of State from  to  Sumner Welles was Under Secretary of State from  to   In  Cordell Hull was Secretary of State nominated confirmed and sworn in In  Sumner Welles had also been nominated confirmed and sworn in as Under Secretary of State In the normal course of events there is only an acting Secretary in the absence of a confirmed nominee eg  L Patrick Gray as acting Director of the FBI after the death of J Edgar Hoover  The Under Secretary of State may act in lieu of the Secretary due to vacation official travel etc but is not normally referred to as acting Secretary anymore than a Lieutenant Governor signing a bill when the Governor is unavailable is the acting Governor heshe is arguably acting for the Governor unless there has been some official transfer of authority as for instance when the person is undergoing surgery  usually that is referred to as temporary acting  if Sumner Welles was Acting Secretary of State there should be some explanation as to why eg in the absence of Cordell Hull on diplomatic mission to