It’s very, very common. For financial aid, you’ll just need to show your documentation to the school’s financial aid office (passport works). As for the code … sometimes I couldn’t figure out why a student got it, but many times, it turned out that they needed to update their citizenship status with the SSA. That apparently is not done automatically when you become a citizen.
(You won’t be able to see the codes that appear on the linked page from @ucbalumnus . Those are available only to the school.)