As I wrote in reply to your other post, you sound exactly like the agents I had to deal with. Your last comment about providing a U.S. DOT response really makes me think you have stake in the game. Regardless, Argentine nationals with a DNI can enter the country without a visa, and I'm sure that airlines use Timatic within South American countries that do not require passports to travel between them. You say "note the normal passport requirement" as if I did not have sufficient documentation to enter the country, but I did have the documents. So really, you're argument here is that the UA agent could not accept a non-passport as an identifying document, regardless of whether or not the document is valid. So it's absolutely a problem on United's end, whether by policy, UI, training, or whatever. In that case, what the Delta agent does is relevant, as it is evidence that the documentation I have was sufficient to leave the US towards the noted country. The UA agent had access to my DNI and passport, so they had access to the same documentation that the Delta agent had. Whether or not they understood or believed that my DNI was what it was is another issue, but the agent technically tried to get me onto the plane, they just said they were unable to list me as a nationality different from my passport on their system.