A single Plesk Onyx server is designated as the management node. The management node serves as a single login point for the administrator and all customers, and as the API entry point. The management node cannot be used for hosting.
One or more Plesk Onyx servers can be added to the infrastructure as service nodes. Service nodes host websites, mail, and databases.
Plesk Multi Server uses load balancing algorithms to determine on which node to provide hosting for each newly created subscription. Each subscription is created entirely on a single node, and cannot be distributed across nodes.
By default, the Round Robin algorithm is used for balancing. You can develop an extension that implements another algorithm, and then use it in Plesk Multi Server by registering so called custom balancer. For details, refer to Plesk Multi Server API Reference.
In the Multi Server infrastructure you can apply server-wide settings to all nodes at once. Learn more in Local and Global Server Settings.