Adding countries to contacts via bulk upload with VANID

Hello! 

I am in a unique situation where we have subscribers from multiple different countries, where we know the country but not their full address. I recently tried to use the bulk upload feature to add countries to the individuals where we know their country of residence but received an error message: Missing values in required field MailingCountryCode. 

I am not sure which Mailing Country Code Every Action uses. Does anyone know if its the two-letter, three-letter or number code so we can update our data and have our contacts be able to be sorted by their country for emailing purposes? 

Thank you!