ScaleSetVM orchestration mode: Virtual machine instances added to the scale set are based on the scale set configuration model. The virtual machine instance lifecycle - creation, update, deletion - is managed by the scale set. It the current default VMSS behavior. (Scale set VMs are created in a single shot). VM (virtual machines) orchestration mode: Virtual machines created outside of the scale set can be explicitly added to the scale set. The orchestration mode VM will only create an empty VMSS without any instances, and you will have to manually add new VMs into it by specifying the VMSS ID during the creation of the VM. (Separately VMs are created and added to scale set later) Reference: https://docs.microsoft.com/en-us/azure/virtual-machine-scale-sets/orchestration-modes