Loadline Zone Seasons terms are used to define Season durations and the Loadline Zones to which they apply. You must have the Loadline Zone Seasons Module Right selected, under Ports and Areas in the Data Center.
On the Data Center menu 
In Voyages, Estimates, and Voyage Fixtures, the Estimated Arrival Date at each Port in the Itinerary combines with any Zone association of the Port and the setup on this form to determine the value in the Loadline column on the Draft/Restrictions tab.