The lord usually appoints a new Lord Commander from one of the current serving members of the Sentinels, though there may have been exceptional cases in which a knight was instantly appointed to the Sentinels as well as the position of Lord Commander.
The Lord Commander wears the same uniform armor as the regular members of the Sentinels, with no difference in appearance or regalia.
The position was presumably founded, along with the Sentinels itself, by Lord Asterion Blackgard, Asterion the Black, who conquered the Borderlands approximately eight thousand years before the War of the Five Kings.
The Lord Commander of the Sentinels also holds a seat on the lord's small council by virtue of his office. His role on the council is to give military advice on land-based warfare.