Yes, it's possible to limit the products that are displayed in the product dropdown on cases based on the order that's associated with the case. You can achieve this by using a filtered lookup on the product field.
Here are the steps to create a filtered lookup:
1. Go to the customizations area of Dynamics 365 CE and select the Case entity.
2. Click on the "Fields" section and select the product field that you want to add the filtered lookup to.
3. In the product field properties, set the "Data Type" to "Lookup" and select the product entity.
4. Click on the "Edit" button next to the "Related Records Filtering" option.
5. In the related records filtering dialog, select the "Only show records where" option and then select the order entity in the first dropdown.
6. In the second dropdown, select the order lookup field that's associated with the case entity.
7. In the third dropdown, select the operator "Equal to" and then select the "Case" entity in the fourth dropdown.
8. In the fifth dropdown, select the lookup field that links the order to the case.
9. Save the changes and publish the customizations.
Once you have completed these steps, the product dropdown on cases will only display products that are related to the order that's associated with the case.
