SDx connects to your DMS using the server-side extensibility framework capability, which sends file-related events to a message queue and waits for the required response when those events have been processed. You need to implement custom event handlers that interact with the specific third-party DMS solution to provide the response that is expected by SDx. The event handlers also need the services logic to determine which external DMS the system is working with at the time, especially if there are multiple DMS repositories integrated with the same SDx system.
For more information on server extensibility events, see Server Extensibility.