The title chief of staff identifies the leader of a complex organization, institution, or body of persons and it also may identify a principal staff officer, who is the coordinator of the supporting staff or a primary aide-de-camp to an important individual, such as a president or a senior military officer.
An aide-de-camp is a personal assistant or secretary to a person of high rank, usually a senior military, police or government officer, a member of a royal family, or a head of state.
A president is the leader of a country or a division or part of a country, typically a republic, a democracy, or a dictatorship.
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In general, a chief of staff provides a buffer between a chief executive and that executive's direct-reporting team.
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The chief of staff generally works behind the scenes to solve problems, mediate disputes, and deal with issues before they are brought to the chief executive.
Often chiefs of staff act as a confidante and advisor to the chief executive, acting as a sounding board for ideas.
Ultimately the actual duties depend on the actual position and the people involved.