sanya,
I guess the other answers are possible, but I think "set off" has a different meaning in your sentence.
set off = located in a place apart
by itself = alone
It was an unusual (aviation) unit, located in a place apart from the others by itself, flying an easily identifiable aircraft.
unit = group in the Air force,
examples,
-The mountain was set off by itself, remote and cold in the distance.
-This truly private ground floor room with a dark cappuccino decor is set off by itself in the Carriage House, found by stepping through a separate garden.
-A cozy pool room set off by itself is a perfect place for a group of your friends to play