Unlike an Ambassador-in-Residence who is usually limited to a country and/or embassy, the Ambassador-at-large is entrusted to operate in several usually neighbouring countries, a Region or sometimes a seat of international organizations like the United Nations/ European Union. In some cases an Ambassador at Large may even be specifically assigned a role to Advise and Assist the State or Government in particular issues. Historically, Presidents or Prime Ministers have designated special diplomatic envoys for specific assignments, primarily overseas but sometimes also within the country as Ambassadors-at-Large.
The US President appointed the first official specifically to bear the title of Ambassador at Large on Mar 12, 1949, under the authority vested in Article II, Section 2 of the U.S to deal with specific foreign policy issues which have been frequently, but not always, spelled out in their commissions.
Examples in the diplomatic corps of the US State Department include(d) :