3-digit or 3-letter code when communicating with Tax office

Dear expert expats:


For a cost claim on my tax return, I am in communication with the Belgian tax office. Apparently, I need to include a 3-digit code in my communication. This code must be somewhere on the tax office letters but I cannot find it anywhere! I read through the letter and understand all it says but I cannot clearly see where this so called 3-digit or 3-letter reference code is.


Can anybody please help?


Thanks!


Br, Desperado