The Staff Aide (sometimes called the Senior Administrative Aide) was the head of the Supreme Chancellor's staff and the Chancellor's deputy in the Office of the Supreme Chancellor. The Staff Aide's duties included addressing the public on behalf of the Chancellor, helping organize the Chancellor's schedules, meeting with other figures on behalf of the Chancellor, taking notes on the Chancellor's meetings and ideas, helping the Chancellor form policy, and the like.
The position was apparently held indefinitely, with one person having the capability to hold the office under successive Chancellors, but it was apparently customary for each Staff Aide to resign after a new Chancellor was elected, so the Chancellor could nominate a Staff Aide of his or her own.
The position of Staff Aide was prestigious enough that the Staff Aide could sit on the upper level of the central Senate pod with the Chancellor and Vice Chair during sessions of the Senate.
Sei Taria was the Staff Aide for the administration of Finis Valorum, retiring shortly after Valorum was replaced by Palpatine. Sly Moore held the position until at least the early days of the Galactic Empire. The continued existence of the position is unknown; it would seem that at some point it was replaced by that of the Grand Vizier.