For lords, there were the public sphere and the private sphere, and Karo managed whole domain administration, while the Sobayonin managed the family affairs of the lord family and played a role of a secretary of the lord and his heir.
藩主にも公私の別があり、藩政を統括したのが家老なら、藩主の家の家政を取仕切ったり、藩主藩主や世子の秘書的な役割を担ったのが側用人だった。