Unlike sales and transfer documents, Business Central does not contain fields for Shipping Agent and Shipping Agent Service on purchase documents, however we may still want to collect purchase orders or ship purchase returns as part of a shipment management route. In this case the fields in the Purchase Collections/Returns tab should be populated. 
When selecting your Collection Shipment Method, you should set it to the Shipment Method Code that will be used on Purchase Orders that will be collected via Shipment Management.
The code should indicate the goods are going to be picked up by as part of a Shipment Management Route.. When a Purchase Order is created, and a Shipment Method is entered that matches the Collection Shipment Method in setup, the purchase order will be added to a Shipment Management Route for collection. When a Purchase Return Order is created, and a Shipment Method is entered that matches the Collection Shipment Method in setup, the purchase return order will be added to a Shipment Management Route for shipment.
You can either type in the code if you know it or select from the drop-down list. The list will show all the shipment methods you have created in Microsoft Dynamics Business Central.
Collection Shipping Agent & Collection Shipping Agent Service Code, these are two fields that you won’t find on the Purchase Order or Purchase Return Order These fields are however required to make sure that your orders are put on the correct routes and schedules for the drivers so that they know a collection as well as a delivery needs to be made. Purchase Orders will be added to Routes using a Delivery Schedule that matches the Shipping Agent and Shipping Agent Code specified here.
Note: If you do not wish to have purchase orders on shipment management routes you can leave these fields blank.