What does “Set as primary identifier” mean in Flatfile Embed?

When a field is designated as a primary identifier, that field acts as a key, or an identifier, for records across multiple uploads. This primary ID allows those records to be updated rather than duplicated. For example, if you give all of your users a Unique User ID, and you set the Unique User ID field as the primary identifier in your data, a user could change their email address without creating a duplicate record in your database.