Steve Endow discovers (rediscovers?) the Inventory transaction “Reason Code” field in GP 2013.
Dynamics GP Land: Inventory transaction “Reason Code” field in GP 2013
Sales, Purchasing & InventoryDevelopment & IntegrationGP 2013
Archived post from 2014. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.