Daniel 6:2

And so Darius, who was called Ahasuerus, received the kingdom and "appointed one-hundred-twenty satraps and appointed three governors over them, whom Daniel was one of, for the satraps to give account that the king may not be overburdened. And Daniel was over them because a spirit abounded in him, and the king appointed him over his whole kingdom." [Daniel 5:31-6:3] For Darius the Mede, after hearing the things concerning him, both the wisdom which he acquired and his faith and piety from childhood, he appointed him to be first of all his friends and to rule all his kingdom, as Pharaoh did, having granted to Joseph second honor of the throne of his kingdom and assigning all the chiefs of the Egyptians to be obedient to him.

Normally, in fact, those entrusted with kingship or any other rule confide least in the people closest to their predecessors; but Daniel enjoyed the same trust from all, receiving equal privileges and being entrusted with the same governance—hence, of course, his becoming one of the three supervisors under Darius. Now, by "supervisors" I think there is reference to what are now called viceroys, and by "satraps" to the governors of the nations, Daniel being one of the supervisors.