The switch '--availability-zone' doesn't work with Azure SQL Database

I have been looking into the possibility of 'pinning' an Azure SQL database to a specific zone and the '--availability-zone' switch for the 'az sql db create' command would appear to indicate that this is possible. However I can't get this to work. The Microsoft docs appear to suggest that the Azure SQL Database service cannot be zonal only zone redundant. If this is so, what is the function of the '--availability-zone' switch ?

https://learn.microsoft.com/en-us/cli/azure/sql/db?view=azure-cli-latest#az-sql-db-create

https://learn.microsoft.com/en-us/azure/reliability/availability-zones-service-support

https://learn.microsoft.com/en-us/azure/azure-sql/database/high-availability-sla-local-zone-redundancy?view=azuresql#high-availability-through-zone-redundancy