The Overture to the Abducation from the Seraglio comes from the opera that was probably Mozart's most popular during his lifetime.  The opera itself has some of the most brilliant and demanding music Mozart ever wrote and the overture is no exception.  Most of the parts are quite demanding right down to the bass instruments.  This arrangement is done for piccolo, Eb flute (alternate C flute part included), 3 C flutes, alto flute, bass flute and contrabass flute.  The contrabass flute part is optional, though it does provide the depth of the string basses.  When playing, it primarily doubles the bass flutes an octave lower much like the string bass does to the cello.