What is IL, YL, OL, OD .......

Yellow letter is a computer generated letter printed on yellow paper (same for everyone regardless) USCIS sent out to some, not all N400 applicants after their biometric before the interview, basically telling them to bring their driver licenses or state issued ID to the interview. So don't panic if you don't get one before getting your interview letter or a white letter (which is a customized letter for N-400 applicants to bring with them a certain documents to the interview, it's generated based on your own situation).

best of luck!
 
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