The availability option you should use for your VMs depends on your image's location. The following table explains the relationship each setting has with these variables to help you figure out which option is best for your deployment.
Availability optionImage location NoneGallery NoneBlob storage Availability zoneGallery (blob storage option disabled) Availability set with managed SKU (managed disk)Gallery Availability set with managed SKU (managed disk)Blob storage Availability set with managed SKU (managed disk)Blob storage (Gallery option disabled) Availability set (newly created by user)Gallery Availability set (newly created by user)Blob storage