This is a drop-down list used to select a Proxmox node to show only the virtual machines in that node. This also sets the task to apply to that node only. For example, if we select a particular node and a VM in it to commit a backup, and we later move that VM to another node, no backup task will be performed since the VM is no longer in the original node. By default, all nodes are selected. In our example, we have selected all nodes.