This field is activated only if you have configured the setting Manage payment plans on purchase orders under Payment plans under the Purchase tab in the Settings procedure.
You can then select a default payment plan for the supplier. This selected payment plan will be used by default when registering purchase orders, but you can always choose to change it on individual orders. If you leave this field empty, the supplier invoice management will be made as usual.
Payment plan templates are configured in the Invoicing / Payment Plans in the Global Settings module.