At Data Automation, we recently built some stuff for use in ShipStation’s “Auctane Fulfillment Network”, as it is called in the API.
It looks like they refer to it as Send Orders To Fulfillment in their documentation

This is a fairly clever innovation where if the Seller has ShipStation, and their Third Party Logistics (3PL) provider also uses ShipStation, they can essentially “Send To Fulfillment”, meaning it makes a copy of the order in the 3PL’s ShipStation account for them to fulfill the order. Once the 3PL fulfills the order, it copies the shipment information, including Carrier, Service, Tracking Number, and Estimated Delivery Date, back to the seller’s ShipStation account.

It looks like it is still a little convoluted to set-up. The 3PL and Seller need to coordinate some things back and forth via email to begin. But once set up, the seller can simply click the “Send to Fulfillment” button inside their ShipStation account to assign the order to their 3PL. You can also set up automation rules to make that happen automatically depending on the sales channel, sku, and other things

From a technical perspective, the order is duplicated into the 3PL’s system, but not quite exactly the same as if it was pulled from the channel directly.

Its always nice when working with a pleasant customer to troubleshoot new things. With their help we got this sorted out and running smoothly now for our Amazon Custom integration with ShipStation at Data Automation