FBA IDs are assigned to every shipment going to a different FBA warehouse.

What is an FBA ID?

FBA IDs are assigned to FBA shipments in Amazon Seller Central after creating a shipment plan. If a shipment is split into multiple fulfillment centers, each portion of the shipment going to a different FBA warehouse will have a different FBA ID:

Provide Flexport with your Amazon shipment’s FBA ID(s) when you submit a quote request.