I believe that admin level 1 should be required for the following countries, even though they are currently optional in the format files.
Australia. See https://auspost.com.au/sending/check-sending-guidelines/addressing-guidelines for the postal guidelines: "The last line should contain the place name or post office of delivery, state or territory abbreviation and postcode. This line should be printed in capitals without punctuation or underlining, with the postcode put last.".
I have also checked every other country in the data set where admin1 is supported but optional. I think it's just the following three, and they look correct to me.
Ireland. See https://correctaddress.anpost.ie/pages/Search.aspx, where the county should be shown on an envelope "where required". That implies it is not always required. As a concrete example where that is true, addresses in Dublin frequently leave off the "Co. Dublin" when formatting the address.
I believe that admin level 1 should be required for the following countries, even though they are currently optional in the format files.
Australia. See https://auspost.com.au/sending/check-sending-guidelines/addressing-guidelines for the postal guidelines: "The last line should contain the place name or post office of delivery, state or territory abbreviation and postcode. This line should be printed in capitals without punctuation or underlining, with the postcode put last.".
Turkey. See https://docs.informatica.com/data-as-a-service/address-verification/h2l/address-verification-best-practices-for-turkey/address-verification-best-practices-for-turkey/key-address-elements.html
I have also checked every other country in the data set where admin1 is supported but optional. I think it's just the following three, and they look correct to me.
Malaysia. The examples on https://www.pos.com.my/business/business-mail-solutions/products/standard-mail.html include some with the province written and some without it.
Argentina. See https://docs.informatica.com/data-as-a-service/address-verification/h2l/address-verification-best-practices-for-argentina/address-verification-best-practices-for-argentina/key-address-elements.html.
Ireland. See https://correctaddress.anpost.ie/pages/Search.aspx, where the county should be shown on an envelope "where required". That implies it is not always required. As a concrete example where that is true, addresses in Dublin frequently leave off the "Co. Dublin" when formatting the address.