What is the relation between Cloud service and VM in Azure

hi

I noticed that when a we create a VM there is a Cloud service is also created. Also we can create multiple VMs under the same Cloud service or each VM can have their own cloud Service.

I read in a link that when VMs are under the same Cloud Service they  "share the workload with existing VM" what does that mean?

Are their any guide lines when to create VMs with seperate Clound Service and when to create VMs under the same Cloud Service.

Thanks