This is needed when you have more that one orchestra definition. It lets you define what orchestra the instrument inheritation will follow. Inheritation follows by default the latest def-orchestra.
(def-orchestra 'orchestra
all-instruments (piano synth)
piano (left-hand right-hand)
)
(def-class symbol piano
all-section '(a -b)
)
Here the instrument inheritation would follow orchestra2, but use-orchestra sets it to follow orchestra1.