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When using the Core Service Scheduler functionality (Not Field Service) you are not able to use Least Busy and Preferred Resource together.

In the Service, resource requirements you can choose a sort option of Least Busy, Most Busy, Randomised and None. We need to use Least Busy to ensure the scheduler offers up the resource that is least busy.

We also use Preferred Resource on the Contact record to indicate the Preferred Resource that the customer works with.

We have found that we cannot use both these together. They should not be mutually exclusive. You should be able to choose Least Busy to schedule the resource with the most free time, but also be able to use Preferred resource where the customer has one indicated.
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