Sometimes countries also appoint highly respected individuals as Ambassador at Large who are assigned specific responsibilities and they work to advise and assist their governments in a given area.
In everyday usage it applies to the ranking government representative stationed in a foreign capital. The host country typically allows the ambassador control of specific territory called an embassy, whose territory, staff, and even vehicles are generally afforded diplomatic immunity in the host country.
The senior diplomatic officers among members of the Commonwealth of Nations are known as High Commissioners, who are the heads of High Commissions. Representatives of the Holy See are known as Papal or Apostolic Nuncios.
Resident Coordinator of UN system are accredited to the Head of State and have the same rank.
Historically, officials representing their countries abroad were termed ministers, but this term was also applied to diplomats of the second rank. The Congress of Vienna of 1815 formalized the system of diplomatic rank under international law:
Ambassadors are ministers of the highest rank, with plenipotentiary authority to represent their head of state. In modern usage, most Ambassadors on foreign postings as head of mission carry the full title of Ambassador Extraordinary and Plenipotentiary. "Ordinary" Ambassadors and non-plenipotentiary status are rarely used, although they may be encountered in certain circumstances.
Moreover, a Resident Ambassador is one who resides within the country to which (s)he is accredited. A Non-Resident Ambassador does not reside within the country to which (s)he is accredited but lives in a nearby country. Thus, a resident ambassador to a country might at the same time also be a non-resident ambassador to one or more other countries; this may be phrased as "Ambassador to" ... "with concurrent accreditation to" another country.
Ambassador-at-Large is a Diplomat of the Highest rank and/ or a Minister who is accredited to represent his country. But unlike the Resident Ambassador (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.
Among European powers, the ambassador extraordinary and plenipotentiary (French ambassadeur extraordinaire et plénipotentiaire or German außerordentlicher und bevollmächtigter Botschafter) was historically deemed the personal representative of the Sovereign, and the custom of dispatching ambassadors to the head of state rather than the government has persisted. For example, ambassadors to and from the United Kingdom are accredited to or from the Royal Court of St. James's (referring to St. James's Palace).
Because 33 members of the Commonwealth of Nations have or had a common head of state, they do not exchange ambassadors, but instead have High Commissioners which represent the government, rather than the head of state. In diplomatic usage, High Commissioner is considered an equivalent rank and role to an Ambassador.
Ranking below full ambassador are the rank of Envoy, Minister resident and Chargé d'affaires. They represent their government rather than their head of state. For further details, see diplomatic rank.
While the title generally reflects the Ambassador's position as head of a diplomatic mission, in some countries the term may also represent a rank held by career diplomats, as a matter of internal promotion, regardless of the posting, and in many national careers it is quite common for them to be appointed to other functions, especially within the ministry/ministries in charge of foreign affairs, in some countries in systematic alteration with actual postings.
The formal form of address for an ambassador is generally the form that would be used to address a head of state: "(Your/His/Her) Excellency" followed by name and/or the country represented. In many countries, less formal variations are frequently used, such as "Ambassador" followed by name, or the name followed by "Ambassador of...". In the United States, "Mr. Ambassador" may be used.
In some countries, a former Ambassador may continue to be styled and addressed as Ambassador throughout his or her life (in the United States, "Mr. Ambassador" or equivalent terms for females may be heard). In other countries, Ambassador is a title that accrues to the individual only with respect to a specific position, and may not be used after leaving the position. Some countries do not use the term while an Ambassador is in the home country, as the individual is not an Ambassador there; for example, a Canadian Ambassador while in Canada is not generally addressed as Ambassador, although he or she may be referred to as "Canadian Ambassador to ...", that is, with reference to a specific job function; they are never addressed or styled as Ambassador after leaving a position or retiring.
In French speaking regions such as France, Wallonia or Quebec, the title of ''ambassadeur person.