They seem to be pretty straightforward and utilitarian: Mexico is MEX, Scotland is SCO, Haiti is HAI, etc. The ones that aren’t the first three letters of the country’s name also make sense: USA for United States of America, for example. I’ve noticed a few outliers, though. Morocco, for instance, is MAR, which I’m guessing has something to do how Morocco’s name of its own country is rendered in Arabic (or maybe Berber) and and then transliterated into Roman script. Bosnia & Herzegovina is BIH, which I’m guessing has to do with the I being the word for “and” in … their language. Or maybe the I in Bosnia. Or something.
So, does FIFA tell each country how to render the letters, in their own tongue? Will the Germans watching see their team rendered as DEU on the chyron on the screen? Or is it consistent across all countries that use Roman script?