just a guess
the highest-ranking Imperial official on a planet, he is the Emperor's liaison with a planet's existing government.
an Imperial governor, presiding over a sector of the galaxy which has been denoted as a priority sector by the Emperor. The Grand Moff's jurisdiction may span one or more sectors, incorporating part of all of the space ruled over by a Moff.
hope i got it right
p.s. example of a prefect: DEFT AKLIN!