One of the use cases I’ve been working on with a customer is based on the vRO/vCO multi-node plug-in and involves the master vRO/vCO node calling proxy workflows based on a parameter - in this case the target site. As you can see from this very simple diagram, a Cloud Management System (CMS) calls a workflow on the Master node, which then executes a proxy workflow on the correct site. The trick is getting the Master Orchestrator node to pick the right proxy workflow.
To quote the release notes for the latest version of vCO/vRO Multi-node Plugin:
The VMware vCenter Orchestrator Multi-Node Plug-In allows organizations to manage environments with multiple vCenter Orchestrator server instances.
As organizations increase their level of automation, they often find the need to deploy multiple Orchestrator instances. With the VMware vCenter Orchestrator Multi-Node Plug-In, administrators have a more efficient way to manage multiple Orchestrator instances from a central point. The plug-in allows administrators to log in to a master Orchestrator server to view the inventories and workflows of remote Orchestrator servers, and to trigger workflows remotely.