Policy on how a vehicle can be unloaded. Applies only to shipments
having both a pickup and a delivery.
Other shipments are free to occur anywhere on the route independent
of unloading_policy.
Values:
UNLOADING_POLICY_UNSPECIFIED (0):
Unspecified unloading policy; deliveries must
just occur after their corresponding pickups.
LAST_IN_FIRST_OUT (1):
Deliveries must occur in reverse order of
pickups
FIRST_IN_FIRST_OUT (2):
Deliveries must occur in the same order as
pickups
