VM Scale Set consists of a set of identically configured VMs.
Availability Set consists of a set of discrete VMs.
No more than 20% of the Scale Set upgrading at any time, then 2 machines out of 10 will have maintenance, the 8 remaining VMs will be up.
Virtual machine scale sets are created with five fault domains by default in Azure regions with no zones. For the regions that support zonal deployment of virtual machine scale sets and this option is selected, the default value of the fault domain count is 1 for each of the zones. FD=1 in this case implies that the VM instances belonging to the scale set will be spread across many racks on a best effort basis.
Reference:
https://docs.microsoft.com/en-us/azure/virtual-machines/manage-availability
https://docs.microsoft.com/en-us/learn/modules/build-app-with-scale-sets/2-features-benefits- virtual-machine-scale-sets
https://docs.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets- automatic-upgrade