Starting June 1, 2019, a new policy will take effect for shipments created through Amazon Marketplace Web Service (Amazon MWS).
Seller will be required to approve all shipments in multi-destination shipping plans and send inventory according to each shipping plan. When sellers use Amazon MWS, the inventory may require multiple shipment destinations to increase the availability of items across Amazon’s fulfillment network. Shipments that comply with FBA policy can be received at fulfillment centers more quickly, and the inventory can be made available for sale sooner. Amazon will notify Sellers by email when shipments don’t comply with FBA policy.
Reference: Shipments(Login required)