Overview
Supplementary items are products that must or can be bought together with other products. Supplementary products might be added free of charge as accompanying products, or you may be able to decide whether to add them to the order or not.
Example
A phone comes with a MicroSD memory card, included free of charge.
In D365
In Dynamics 365 Finance and Supply Chain Management, products can be set up to include supplementary items. This is to propose products that are related to the product that’s being ordered. The additional products may be required, or may be optional. In some cases, supplementary items may be added as free products that accompany the purchase of other products.
This can be configured as part of creating a purchase order. Review and add relevant supplementary items for all the order lines by using the Supplementary items page, which you can open from the Action Pane.

To add supplementary items to a purchase order, first, you need to build a relationship between the main item and supplementary item. This relationship can be set up using Supplementary purchase items on the Released product form.
To add a supplementary item, open the Released product details form in the Product information management module, and on the Purchase tab click Supplementary purchase items. Use this form to add a supplementary item and build a relationship with the main item.
